Definition of "quag"

If a person ventures on one of these quags, it bends in waves under his feet; and if the surface breaks, he is in danger of sinking to the bottom.

1771 December 16, John Walker, Account of the Irruption of Solway Moss

Crooked or straight, through quags or thorny dells

1784, William Cowper, Tirocinium; or, a Review of Schools