before vowels pycn-, word-forming element meaning "close, thick, dense," from Latinized combining form of Greek pyknos "thick, dense, solid, compact; numerous; strong; brave; clever," which is of uncertain origin. Often in English via German as pykno-.
word-forming element in modern science meaning "seaweed, algae," from Latinized form of Greek phykos "seaweed, sea wrack," also "rouge, red make-up made from seaweed;" Beekes writes that it is a loan-word from Semitic and compares Hebrew pūk "eye-rouge." "The meaning 'make-up' is therefore primary for [phykos], too; hence 'seaweed'." Compare fucus, which is probably a Latin borrowing of the Greek word.
before vowels scler-, word-forming element meaning "hard," from Latinized form of Greek sklēros "hard, harsh," which is related to skellein "to dry up, parch" (from PIE *skle-ro-, from root *skele- "to parch, wither").
fem. suffix, from French -esse, from Late Latin -issa, from Greek -issa (cognate with Old English fem. agent suffix -icge); rare in classical Greek but more common later, in diakonissa "deaconess" and other Church terms picked up by Latin.
standard ending in chemical names of sugars, originally simply a noun-forming suffix, taken up by French chemists mid-19c.; it has no etymological connection with sugar. It appears around the same time in two chemical names, cellulose, which would owe it to the French suffix, and glucose, where it would be a natural result from the Greek original. Flood favors origin from glucose.
occasional plural suffix of words ending in -a (see a- (1)), most of which, in English, are from Latin nominative fem. singular nouns (or Greek ones brought up through Latin), which in Latin form their plurals in -ae. But plurals in native -s were established early in English for many of them (such as idea, arena) and many have crossed over since. Purity now would only breed monsters.
word-forming element making nouns of quality, state, or condition, from Middle English -our, from Old French -our (Modern French -eur), from Latin -orem (nominative -or), a suffix added to past participle verbal stems. Also in some cases from Latin -atorem (nominative -ator).
In U.S., via Noah Webster, -or is nearly universal (but not in glamour), while in Britain -our is used in most cases (but with many exceptions: author, error, tenor, senator, ancestor, horror etc.). The -our form predominated after c. 1300, but Mencken reports that the first three folios of Shakespeare's plays used both spellings indiscriminately and with equal frequency; only in the Fourth Folio of 1685 does -our become consistent.
A partial revival of -or on the Latin model took place from 16c. (governour began to lose its -u- 16c. and it was gone by 19c.), and also among phonetic spellers in both England and America (John Wesley wrote that -or was "a fashionable impropriety" in England in 1791).
Webster criticized the habit of deleting -u- in -our words in his first speller ("A Grammatical Institute of the English Language," commonly called the Blue-Black Speller) in 1783. His own deletion of the -u- began with the revision of 1804, and was enshrined in the influential "Comprehensive Dictionary of the English Language" (1806), which also established in the U.S. -ic for British -ick and -er for -re, along with many other attempts at reformed spelling which never caught on (such as masheen for machine). His attempt to justify them on the grounds of etymology and the custom of great writers does not hold up.
Fowler notes the British drop the -u- when forming adjectives ending in -orous (humorous) and derivatives in -ation and -ize, in which cases the Latin origin is respected (such as vaporize). When the Americans began to consistently spell it one way, however, the British reflexively hardened their insistence on the other. "The American abolition of -our in such words as honour and favour has probably retarded rather than quickened English progress in the same direction." [Fowler]