fifth (adj., n.)

"next in order after the fourth; an ordinal numeral; being one of five equal parts into which a whole is regarded as divided;" c. 1200, fift, from Old English fifta "fifth," from Proto-Germanic *finftan- (source also of Old Frisian fifta, Old Saxon fifto, Old Norse fimmti, Dutch vijfde, Old High German fimfto, German fünfte, Gothic fimfta),from *fimfe "five" (from PIE root *penkwe- "five") + *-tha, the word-forming element used to make ordinal numbers (see -th (1)). Normal development would have yielded fift; altered 14c. by influence of fourth.

Noun meaning "fifth part of a gallon of liquor" is first recorded 1938, American English; the noun in the music sense is from 1590s. Fifth Avenue (in New York City) has been used figuratively for "elegance, taste" at least since 1858. Fifth wheel "superfluous person or thing" attested from 1630s. It also was the name of a useful device, "wheel-plate or circle iron of a carriage" placed on the forward axle for support and to facilitate turning (1825). And the phrase sometimes is turned on its head and given a positive sense of "that which a prudent driver ought to take with him in case one of the others should break" (1817). Fifth-monarchy-man, 17c. for "anarchist zealot," is a reference to Daniel ii.44.