grub (v.)
c.1300, from hypothetical Old English *grubbian, from West Germanic *grubbjan (cognates: Middle Dutch grobben, Old High German grubilon "to dig, search," German grübeln "to meditate, ponder"), from Proto-Germanic *grub- "to dig," base of Old English grafan (see grave (v.)).
grub (n.)
"larva," early 15c., perhaps from grub (v.) on the notion of "digging insect," or from the possibly unrelated Middle English grub "dwarfish fellow" (c.1400). Meaning "dull drudge" is 1650s. The slang sense of "food" is first recorded 1650s, said to be from birds eating grubs, but also often linked with bub "drink."