Etymology
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pn- 

consonant sound in some English words derived from Greek. The p- typically is silent in English but pronounced in French, German, Spanish, etc.

It is to be desired that it were sounded in English, also, at least in scientific and learned words; since the reduction of pneo- to neo-, pneu- to new-, and pnyx to nix, is a loss to etymology and intelligibility, and a weakening of the resources of the language. [OED]
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step- 
Old English steop-, with connotations of "loss," in combinations like steopcild "orphan," related to astiepan, bestiepan "to bereave, to deprive of parents or children," from Proto-Germanic *steupa- "bereft" (source also of Old Frisian stiap-, Old Norse stjup-, Swedish styv-, Middle Low German stef-, Dutch stief-, Old High German stiof-, German stief-), literally "pushed out," from PIE *steup-, from root *(s)teu- (1) "to push, stick, knock," with derivatives referring to fragments (see steep (adj.)). Barnhart suggests the forms in -f- are by assimilation of the first sound in following words for "father."

Etymologically, a stepfather or stepmother is one who becomes father or mother to an orphan, but the notion of orphanage faded in 20c. and came to denote simply relation through marriage. For sense evolution, compare Latin privignus "stepson," related to privus "deprived." Compare orphan (n.).
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e- 

the later Romans evidently found words beginning in sc-, sp-, st- difficult or unpleasant to pronounce; in Late Latin forms begin to emerge in i- (such as ispatium, ispiritu), and from 5c. this shifted to e-. The development was carried into the Romanic languages, especially Old French, and the French words were modified further after 15c. by natural loss of -s- (the suppression being marked by an acute accent on the e-), while in other cases the word was formally corrected back to the Latin spelling (for example spécial). Hence French état for Old French estat for Latin status, etc. It also affected Romanic borrowings from Germanic (such as espy, eschew).

A different e- is a reduced form of Latin ex- before consonants (see ex-), and the e- in enough is an unfelt survival of an Old English alternative form of ge-.

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for- 

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.

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