Английский Этимологический словарь - grub (v.)

- early 14c., from hypothetical O.E. *grubbian, from W.Gmc. *grubbjan, from P.Gmc. *grub- "to dig," base of O.E. grafan (see grave (v.)). The noun sense of "larva" is perhaps from the notion of "digging insect," or from the possibly unrelated M.E. grub "dwarfish fellow." The slang sense of "food" is first recorded 1659, said to be from birds eating grubs, but also often linked with bub "drink." Grubby is pre-1845, from grub (n.) in a sense of "dirty child," who presumably got that way from digging in earth.