A snag is something sharp that sticks out, like a splinter or a dead tree branch. It’s also a hitch in a plan. If you develop a terrible cat allergy, your lifelong dream of being a cat trainer has hit a snag.
A snag is something that you get stuck on, either literally or figuratively. If you catch your tights on a snag and change into different ones, it might put a snag in your plans to dress all in pink for the day. When something's caught this way, it snags. Snag, a U.S. coinage, was first used when steamboats got stuck on a log or branch in the river: "Captain, we've hit a snag!"