Fleming
late Old English Flæmingi, from Old Norse, reinforced by Middle Dutch Vlāming, related to Vlaanderen ‘Flanders’.
wiktionary
From Middle English flameng, from late Old English Flæmingi, from Old Norse Flǽmingr, Middle Dutch Vlaminc, Vleminc, from Old Frisian (unattested) (compare West Frisian Flaming), from Proto-Germanic *flaumaz(“flowing, current (water)”) and the patronymic suffix *-ingaz(“belonging to, descended from”). More at Flanders.
etymonline
Fleming (n.)
from Old English Flæming "native or inhabitant of Flanders," from Old Dutch Vlaemingh, Old Frisian Fleming, both from Proto-Germanic *Flam- (see Flanders). The Germanic name was borrowed in Medieval Latin as Flamingus, hence Spanish Flamenco, Provençal Flamenc, etc. French has flandrin "a lanky lad" (15c.), originally a nickname of a Fleming, thence "any tall and meagre man," as they were thought to be [Kitchin].
