sucker (n.)
"young mammal before it is weaned," late 14c., agent noun from suck. Slang meaning "person who is easily deceived" is first attested 1836, American English, on notion of naivete; but another theory traces the slang meaning to the fish called a sucker (1753), on the notion of being easy to catch in their annual migrations (the fish so called from the shape of its mouth). As a type of candy from 1823; especially "lollipop" by 1907. Meaning "shoot from the base of a tree or plant" is from 1570s. Also the old name of inhabitants of Illinois.
sucker (v.)
"to deceive, to make a dupe of," 1939, from sucker (n.) in the related sense. Related: Suckered; suckering.