prefix usually meaning "away, opposite, completely," from Old English for-, indicating loss or destruction, but in other cases completion, and used as well with intensive or pejorative force, from Proto-Germanic *fur "before, in" (source also of Old Norse for-, Swedish för-, Dutch ver-, Old High German fir-, German ver-); from PIE *pr-, from root *per- (1) "forward," hence "in front of, before, toward, near, against."
In verbs the prefix denotes (a) intensive or completive action or process, or (b) action that miscarries, turns out for the worse, results in failure, or produces adverse or opposite results. In many verbs the prefix exhibits both meanings, and the verbs frequently have secondary and figurative meanings or are synonymous with the simplex. [Middle English Compendium]
Probably originally in Germanic with a sense of "forward, forth," but it spun out complex sense developments in the historical languages. Disused as a word-forming element in Modern English. Ultimately from the same root as fore (adv.). From its use in participles it came to be an intensive prefix of adjectives in Middle English (for example Chaucer's forblak "exceedingly black"), but all these now seem to be obsolete.
before vowels, cephal-, word-forming element meaning "head, skull, brain," Modern Latin combining form of Greek kephalē "head, uppermost or top part, source," from PIE *ghebh-el- (source also of Tocharian spal "head;" Old High German gebal "skull;" also, via the notion of "front," Gothic gibla, Old Norse gafl "side of a facade").
word-forming element meaning "horn, horn-like part," from Latinized form of Greek keras (genitive keratos) "horn of an animal; horn as a substance," from PIE root *ker- (1) "horn, head."
word-forming element meaning usually "one half more" than the indicated unit, from Latin sesqui-, sesque- "one and a half; one-half more," contraction of *semis-que- "a half in addition," from *semis "a half" (see semi-) + -que "and" (from PIE root *kwe "and, -ever," for which see ubiquity). Latin semi- had a tendency to get partly swallowed in compounds; compare these derivatives listed in de Vaan: selibra "half a libra," semodius "half a modius," sestertius "having the value of two-and-a-half" (as a noun, the name of a silver Roman coin, short for sestertius nummus), contracted from *semistertius; simbella "coin worth half a libella;" sincipitis "a half-head."
word-forming element meaning "man, male, masculine," from Greek andro-, combining form of anēr (genitive andros) "a man, a male" (as opposed to a woman, a youth, or a god), from PIE root *ner- (2) "man," also "vigorous, vital, strong."
Equivalent to Latin vir (see virile). Sometimes in later use equivalent to anthrōpos, Latin homo "a person, a human being," and in compounds it often retain this genderless sense (e.g. androcephalous "having a human head," said of monsters including the Sphinx, which in Greece was female).