Etymology
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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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plunk (v.)

1805, "to pluck a stringed instrument;" 1808 in sense of "drop down abruptly;" 1888 as "to hit, wound, shoot." Probably of independent imitative origin in each case. Related: Plunked; plunking.

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for (prep.)
Old English for "before, in the sight of, in the presence of; as far as; during, before; on account of, for the sake of; in place of, instead of," from Proto-Germanic *fur "before; in" (source also of Old Saxon furi "before," Old Frisian for, Middle Dutch vore, Dutch voor "for, before;" German für "for;" Danish for "for," før "before;" Gothic faur "for," faura "before"), from PIE root *per- (1) "forward," hence "in front of, before," etc.

From late Old English as "in favor of." For and fore differentiated gradually in Middle English. For alone as a conjunction, "because, since, for the reason that; in order that" is from late Old English, probably a shortening of common Old English phrases such as for þon þy "therefore," literally "for the (reason) that."
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go for (v.)
1550s, "be taken or regarded as," also "be in favor of," from go (v.) + for (adv.). Meaning "attack, assail" is from 1880. Go for broke is from 1951, American English colloquial.
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good-for-nothing (adj.)
"worthless," 1711, from adjectival phrase (see good (adj.)).
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tit for tat 
1550s, possibly an alteration of tip for tap "blow for blow," from tip (v.3) "tap" + tap "touch lightly." Perhaps influenced by tit (n.2).
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free-for-all (n.)
"mass brawl" (one in which all may participate), 1918, from earlier adjective use (1868), especially in reference to open horse races, American English. Earlier as a noun in reference to free-for-all horse and motorcar races.
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plumb (adj.)

"perpendicular, vertical, true according to a plumb-line," mid-15c., plom, from plumb (n.). As an adverb, "in a vertical direction, straight down," c. 1400. The notion of "exact measurement" led to the extended adverbial sense of "completely, downright" (1748), sometimes spelled plump, plum, or plunk.

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main line (n.)

"principal line of a railway," 1841, from main (adj.) + line (n.). Meaning "affluent area of residence" is by 1917, originally (with capitals) that west of Philadelphia, from the "main line" of the Pennsylvania Railroad which added local stops to a string of backwater towns west of the city late 19c. that helped turn them into fashionable suburbs.

The Main Line, Philadelphia's most famous suburban district, was deliberately conceived in the 1870's and 1880's by the [Pennsylvania] Railroad, which built high-toned housing developments, ran hotels, more or less forced its executives to plunk their estates out there, and created a whole series of somewhat spurious Welsh towns along the railroad tracks. ... Now everybody assumes these all date from 1682, like the Robertses; but as Chestnut Hill people like to say, "nobody but Welsh peasants lived on the Main Line till the Railroad built it up." [Nathaniel Burt, "The Perennial Philadelphians," 1963]

The original station stops were, in order out from the city, Overbrook, Merion, Narberth, Wynnewood, Ardmore, Haverford, Bryn Mawr, Paoli. The train line for commuters along it is the Paoli Local.

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death-wish (n.)

"conscious or unconscious desire for death for oneself or for another," 1896, from death + wish (n.).

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