nitpicker n.

"louse egg," Middle English nite, from Old English hnitu, from Proto-Germanic *hnitu- (source also of Norwegian nit, Middle Dutch nete, Dutch neet, Middle High German niz, German Niß), from PIE root *knid- "egg of a louse" (source also of Russian, Polish gnida, Czech knida; Greek konis, genitive konidos "egg of a louse").

early 13c., picken "to peck;" c. 1300, piken "to work with a pick, to dig up," probably representing a fusion of Old English *pician "to prick," (implied by picung "a piercing, pricking," an 8c. gloss on Latin stigmata) with Old Norse pikka "to prick, peck," both from a Germanic root (source also of Middle Dutch picken, German picken "to pick, peck"), perhaps imitative. Influence from Middle French piquer "to prick, sting" (see pike (n.1)) also is possible, but that French word generally is not considered a source of the English word. Related: Picked; picking.

Meaning "to pluck with the hand or fingers, gather, break off, collect" (fruit, etc.) is from early 14c.; that of "to prick or pierce with a pointed instrument" also is from early 14c. The meaning "to choose, sort through carefully in search of valuable material" emerged late 14c., from the earlier meaning "to pluck with the fingers." The sense of "to rob, plunder" (c. 1300) weakened to a milder sense of "steal petty things, filch or pilfer from" by late 14c.  Meaning "to eat with small bites" is from 1580s.

Of locks, etc., "probe or penetrate with a pointed tool," early 15c. The meaning "to pluck (a banjo, etc.) with the fingers" is recorded from 1860. To pick a quarrel, fight, etc. is from mid-15c.; to pick at "annoy with repeated fault-finding" is from 1670s. To pick on "single out for adverse attention" is from late 14c. Also see pick up.

To pick off "shoot one by one" is recorded from 1810; baseball sense, of a pitcher or catcher, "to put out a runner caught off base" is by 1939. To pick and choose "select carefully" is from 1660s (choose and pick is attested from c. 1400). To pick (one's) nose is by mid-15c.

also nit-pick, "seek petty faults," 1962, a back-formation from earlier nitpicker. Related: Nitpicking (1956); nitpicky.