QUAGMIRE, a bog or marsh, a piece of ground so saturated with water that it cannot support any weight. The word is composed of “quag” or “quake” (O.E. cwacian; cf. “quaver,” “quiver”) and “mire,” mud (Icel. myri, Swed. myr).
Skeat suggests that quag may be connected with the root seen in “quick,” and quotes (Etym. Dict. 1898) Piers Plowman, c. xxi. 64, of an earthquake, the earth “quook as it quyke were,” i.e. shook as if it were alive.