1560s, "easily crumbled;" 1570s, "like bread," from crumb. The second sense probably accounts for 18c. (and later in dialects) use, of a woman, "attractively plump, full-figured, buxom." Slang meaning "shoddy, filthy, inferior, poorly made" in use by 1859, probably is from
the first sense, but influenced by crumb in its slang sense of "louse."