"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.
"to deceive, to make a dupe of," 1939, from sucker (n.) in the related sense. Related: Suckered; suckering.