"wet, soft, spongy ground with soil chiefly composed of decaying vegetable matter," c. 1500, from Gaelic and Irish bogach "bog," from adjective bog "soft, moist," from Proto-Celtic *buggo- "flexible," from PIE root *bheug- "to bend." Bog-trotter applied to the wild Irish from 1670s.
A bog is characterized by vegetation, decayed and decaying, and a treacherous softness. A quagmire or quag is the worst kind of bog or slough; it has depths of mud, and perhaps a shaking surface. A slough is a place of deep mud and perhaps water, but generally no vegetation. [Century Dictionary]