Quagmire comes from quag plus mire. Mire, from an Old English word meaning marsh, dates back to the 14th century. Initially, mire named wet, spongy earth (such as that of a bog or marsh), or any heavy, often deep, mud or slush. The literal meaning of mire later turned metaphoric and came to name a situation troublesome or intractable.

So what's with the quag in quagmire?

Appropriately enough, the story of quag is a bit murky. In fact, quag and quagmire both made their initial print appearance during the late 1500s. Etymologists believe the quag in quagmire was probably borrowed from the quag that meant "marsh" or "bog." So where did that quag come from? No one knows for certain, but we do know that quag also once enjoyed a (now archaic) verb sense (probably onomatopoeic in origin) that meant "quake, quiver, or shake."