Maggot
late Middle English: perhaps an alteration of dialect maddock, from Old Norse mathkr, of Germanic origin.
wiktionary
From Middle English magot, magotte, probably Anglo-Norman metathetic alteration of maddock(“worm", "maggot”), originally a diminutive form of a base represented by Old English maþa (Scots mathe), from Frankish *maþō, from common Proto-Germanic *maþô, from the Proto-Indo-European root *mat, which was used in insect names, equivalent to made + -ock. Near-cognates include Dutch made, German Made and Swedish mask.
The use of maggot to mean a fanciful or whimsical thing derives from the folk belief that a whimsical or crotchety person had maggots in their brain.
etymonline
maggot (n.)
the worm or grub of various insects (especially a fly), formerly supposed to be generated by corruption, late 15c., magat, probably an unexplained variant of Middle English maddok, maðek "earthworm, bedbug, maggot," from Old English maða "maggot, grub," from Proto-Germanic *mathon (source also of 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. Hence maggotry "folly, absurdity" (1706).
