"dirty," by 1845, from grub (n.) in a sense of "dirty child" (who presumably got that way from digging in earth) + -y (2). Earlier it was used in a sense of "stunted, dwarfish" (1610s) and "infested with grubs" (1725). Related: Grubbily; grubbiness.[grubby etymology, grubby origin, 英语词源]