Lollipops or candies on a stick, or foolish people who are easily deceived.
From 'suck' (Old English 'sucan') plus '-er' suffix. Originally referred to something that sucks, then applied to the candy, and metaphorically to gullible people.
The word 'sucker' for a gullible person comes from the fish of the same name—bottom-feeders that mindlessly suck up whatever is on the ocean floor, making them seem not very smart!
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