c. 1600, "quality of being very close or compact," from French densité (16c.), from Old French dempsité (13c.), from Latin densitas "thickness," from densus "thick, dense" (see dense). In physics, "the mass of matter per unit of bulk," 1660s.
13th letter of the English alphabet, from Greek mu, from Semitic mem. It represents a very stable and unchanging sound in Indo-European, described by Johnson as "a kind of humming inward." The Roman symbol for 1,000; sometimes used in this sense in English 15c.-16c.; but in late 20c. newspaper headlines it stands for million. As a thickness of type, from 1680s (commonly spelled out, em).
1540s, "thick, coarse, gross, not thin or fine," from French crasse (16c.), from Latin crassus "solid, thick, fat; dense," which is of unknown origin.
The literal sense always has been rare in English. The meaning in reference to personal qualities, etc., "grossly stupid, obtuse" is recorded from 1650s, from French. Middle English had cras (adj.) "slow, sluggish, tardy" (mid-15c.), also crassitude "thickness." Related: Crassly; crassness.