cut to the quick

Old English cwic "living, alive, animate," and figuratively, of mental qualities, "rapid, ready," from Proto-Germanic *kwikwaz (cf. Old Saxon and Old Frisian quik, Old Norse kvikr "living, alive," Dutch kwik "lively, bright, sprightly," Old High German quec "lively," German keck "bold"), from PIE root *gweie- "to live" (see bio-). Sense of "lively, swift" developed by late 12c., on notion of "full of life."

Of persons, "mentally active," from late 15c. Also in Middle English used of soft soils, gravel pits, etc. where the ground is shifting and yielding (mid-14c., cf. quicksand). As an adverb from c.1300. To be quick about something is from 1937. Quick buck is from 1946, American English. Quick-change artist (1886) originally was an actor expert in playing different roles in the same performance of a show. Quick-witted is from 1520s.