Sucker punch seems to mean "an unexpected punch" in slang. What is the origin of this term and why does sucker mean unexpected in sucker punch?
The sucker here has a much longer history than some would have you believe. Basically, sucker punch derives from one of the meanings of the word sucker (NOAD):
Therefore, a "sucker" would be easy to fool with a surprise punch. Etymonline dates the use of the term to 1836, and it appears to derive from the expression for a young mammal before it is weaned, i.e. one that is still sucking at its mother's teat.
Note that despite what Thursagen states, Etymonline also gives different dates for the term suckerpunch:
Its origin is in boxing,:
Here states the exact date:
Why? Being coined by Jack Dempsey who just called it a sucker punch for his reasons, I think it'd be hard to find out. Try asking Jack.