Literally, a sucker is “one who or that which sucks.”
In this sense, one application is to the not-yet-weaned young of any animal, and it is this application that has led to the figurative meaning of “dupe, simpleton.”
For the greenhorns, those “not yet dry behind the ears,” are by this term endowed with the ingenuousness and naïveté that are presumably characteristic of a babe too young to be removed from his mother’s breast.
