"most important piece or feature," 1831, from French pièce de résistance, originally "the most substantial dish in a meal." Literally "piece of resistance;" there seems to be disagreement as to the exact signification.
1961, "comedy that deals in themes and subjects usually regarded as serious or taboo," from black (adj.), in a figurative sense of "morbid," + comedy. Compare French pièce noire, also comédie noire "macabre or farcical rendering of a violent or tragic theme" (1958, perhaps the inspiration for the English term) and 19th-century gallows-humor. In a racial sense, from 1921.
a phrase that stands for "absurd etymology," or generally "anything illogical, outrageous hypothesis," 1711, from the Latin phrase, taken as the outstanding example of such an error.
"A grove (lucus) [is so called] from not (a non) being light" (lucendo, ablative of lucere "to shine;" see light (n.)). That is, it is called a grove because light doesn't get into it. This explanation is found in a commentary on Virgil (Aeneid 1.22) by Servius, a 4th century grammarian, among other places. Other ancient grammarians (notably Quintilian) found it paradoxical and absurd, based on nothing more than the similarity in sound between the two words.
Modern scholarship, however, concludes that lucus and lucere probably do come both from the same PIE root (*leuk-) meaning "light, bright." De Vaan writes: "Lucus 'sacred grove, wood,' from PIE *louk-o- 'light place,' with cognates in Sanskrit loka- 'free space, world,' Lithuanian laukas 'field, land,' Latvian lauks 'field, clearing in the woods,' Old High German loh 'clearing' and English lea 'open field, meadow, piece of untilled grassy ground.' " Apparently the primeval notion in *louk-o- was a lighter place in a thick forest. Migration, change of climate, or felling of the woods might have shifted the meaning.