"lump, quantity, size," late 14c., from Old French masse "lump, heap, pile; crowd, large amount; ingot, bar" (11c.), and directly from Latin massa "kneaded dough, lump, that which adheres together like dough," probably from Greek maza "barley cake, lump, mass, ball," related to massein "to knead," from PIE root *mag- "to knead, fashion, fit." Sense extended in English 1580s to "a large quantity, amount, or number." Strict sense in physics is from 1704.
As an adjective from 1733, first attested in mass meeting in American English. mass culture is from 1916 in sociology (earlier in biology); mass hysteria is from 1914; mass media is from 1923; mass movement is from 1897; mass production is from 1920; mass grave is from 1918; mass murder from 1880.