The oldest words in the English language are around 14,000 years old, originating in a pre-Indo-European language group called Nostratic ("our language") by experts. Words from this language group that survive in modern English include apple (apal), bad (bad), gold (gol), and tin (tin). (source)

The word arctic is derived from the ancient Greek word for bear, arktos. The reason is that the constellation of Ursa Major, the Great Bear, lies in the northern sky. (source)

No English words rhyme fully with orange, silver, or month (there are, however, some partial rhymes, or pararhymes, for these words, such as salver for silver and lozenge for orange). (source)

The longest English word that contains neither A, E, I, O, nor U is rhythms. (source)

In Old English, the word with meant "against". This meaning is still preserved in phrases such as "to fight with". (source)

In English, the days of the week are named after the Saxon gods (except for Saturday, which is named after the Roman god of agriculture). Sunday is named after the sun, Monday after the moon, Tuesday after Tiw, Wednesday after Woden, Thursday after Thor, Friday after Frige, and Saturday after Saturn. (source)

The word boycott comes from Charles C. Boycott. He was hired by an Irish earl to collect high rents from tenant farmers who completely ignored him. (source)

Part of a Roman soldier's pay was called salarium argentium, "salt money", which was used to buy the then-precious commodity, and so pay today is called a "salary". (source)

The word typewriter is one of the longest that can be typed using only the top row of a standard QWERTY keyboard. Others are perpetuity, proprietor, and repertoire and, if you include obscure words, the longest is rupturewort. The longest words that can be typed using only the home row are alfalfas and, counting obscure words, haggadahs and halakhahs. No words can be typed using only the bottom row, because that row contains no vowels. (source)

The longest words that can be typed on a standard QWERTY keyboard using only the left hand are twelve letters long. There are six such words: aftereffects, desegregated, desegregates, reverberated, reverberates, and stewardesses. (source)

The word "mile" comes from the Roman milia, "thousands". The Romans measured distances in paces (from left foot to left foot; we would regard these as double paces), which were about five feet. So, milia passuum, 1,000 paces or about 5,000 feet, was the length of a mile. (source)

The word slave comes from Slav, the name of a group of Eastern European peoples. In antiquity, Germanic tribes captured Slavs and sold them as slaves to Romans. The Latin word for slave, addict, has become the English word for someone dependent on something harmful. (source)

The 1934 edition of Webster's Second New International Dictionary defined the word "Dord" as "density." It turns out that this was a typo; in the publisher's files was the abbreviation for density, "D or d", but somehow the spaces between the letters were lost. The mistake, while spotted in 1939, was not corrected until 1947. (source)

The verb "cleave" has two opposite meanings. It can mean to adhere or to separate.

"Journal" does not have any letters in common with the Latin word from which it is derived: dies, "day." Intermediate steps in the word's development include the Latin diurnus, the Italian giorno, and the French jour. (source)

The quark, a building block of the proton, got its name from James Joyce's Finnegans Wake, from the line "Three quarks for Muster Mark! Sure he hasn't got much of a bark". (source)

The word "uptown" was in use before the word "downtown" was. Both words were originally used to describe parts of Manhattan. (source)

The words "beef" and "cow" come from the same Indo-European root.

Until the seventeenth century the word "upset" meant to set up (i.e. erect) something. Now it means the opposite: "to capsize". (source)

Before 1974, a billion in the United States of America was different from a billion in Great Britain. An American or short scale billion was a thousand million (1,000,000,000), but a British or long scale billion was a million million (1,000,000,000,000). Other names for large numbers also differed between the two countries. Starting in 1974, however, the short scale numbers started to be used exclusively in Great Britain. The original usage is the former British usage (around 1484, Nicolas Chuquet invented the words billion through nonillion to denote the second through ninth powers of a million, while around the middle of the seventeenth century, French arithmeticians began using these words to denote the third through tenth powers of a thousand). (source)

According to the third edition of The Official Scrabble Players Dictionary, there are 20 valid words that contain no vowels. (source)

The word "kindergarten" comes from the German for "children's garden". Friedrich Froebel, who coined the term, originally was planning to use the term "Kleinkinderbeschäftigungsanstalt" instead. (source)

"Dreamt" is the only common English word ending in "mt". The only not-so-common ones are two related words, "adreamt" and "undreamt". (source)

The word "idiot" was once used to describe an ordinary person; it gradually came to mean a layman, as contrasted with a clergyman. Since few outside the clergy were educated, the term became associated with an uneducated and hence ignorant and foolish one, and eventually became associated with a mentally deficient person. (source)

The largest number in the English language with a word naming it is a googolplex. This number is equal to 10 to the power of a googol, or 10 to the power of 10¹⁰⁰. This number would be written as 1 followed by 10¹⁰⁰ zeroes (except that, as there are far fewer particles in the universe than there are zeroes in a googolplex, the number could never be written out in full). The names "googol" and "googolplex" were both suggested in the 1930s by Milton Sirotta, the nine-year-old nephew of mathematician Dr. Edward Kasner. (source)

Acronyms (abbreviations pronounced as words, as opposed to initialisms, abbreviations pronounced as individual letters) were quite rare before the 20th century; the word "acronym" itself only dates from 1943. All stories about the origin of words (including "tip", "posh", "golf", and various four-letter words) that claim that a word is derived from an acronym several centuries old are false.

In the 15th century, the word "prevent" meant to act in anticipation of some occurrence. By the 16th century, its sense had shifted to mean, "to keep from happening." (source)

The first use of the word "robot" to describe advanced humanlike machines was in 1920, in R.U.R., an early science fiction play. It comes from the Czech word robota, meaning "compulsory labour". (source)

The word "tragedy" is derived from two Greek words meaning "goat song".

The word "abracadabra" originated in Roman times as part of a prayer to the god Abraxas, found in a medical work by Quintus Serenus Sammonicus around 250 A.D. Sammonicus is also known for writing his medical works in verse.