maggot (n.)
late 15c., probably an unexplained variant of Middle English maðek, from Old English maða "maggot, grub," from Proto-Germanic *mathon (cognates: Old Norse maðkr, Old Saxon matho, Middle Dutch, Dutch made, Old High German mado, German Made, Gothic maþa "maggot"). Figurative use "whim, fancy, crotchet" is 1620s, from the notion of a maggot in the brain.