quag

Uncertain. Most often suggested to be an alteration of Middle English quabbe (“a marsh, bog”), from Old English *cwabbe (“that which shakes or trembles, something soft and flabby”), from Proto-West Germanic *kwabbā (“soggy ground”); this is supported by 1590s attestations of quabmire for quagmire (other earlier variants are quamire, from the 1550s, and quavemire from the 1520s). It has alternatively been suggested to be related to quake, as quaggy ground quakes when trod. Compare typologically Russian тряси́на (trjasína) (see for more).

来源：wiktionary