I get a daily trivia e-mail from Mailbits.com, and I thought I'd start posting some of these tidbits here, with due credit, of course. Some of them are really pretty interesting. I even have a title for these posts.

Well, Whadya Know

Why is an easy mark a "sucker?"

Do you like fish tails . . . uh, tales? Bear with me. It
begins with the bottom-scavenging fish known as the sucker,
from the way it purses its "lips" to draw in the garbage it
eats (how low can you sink?). The early settlers of America
saw lots of these fish and soon they were indiscriminately
using the name as well for other fish that made the same
motion with their mouths. Eventually so many kinds of fish
bore this name that if you baited a hook you were likely to
haul in a sucker.

Suckers, being so easy to catch, became synonymous with an
easy mark. Any person who would figuratively go for the bait
was also labeled a sucker.

[source: Why You Say It by Webb Garrison]