A pigeon in poker is a player who's seen as a bad player by other players. People may refer to a certain player as a "pigeon" when they think someone doesn't grab the basics of poker strategy or plays a completely different strategy than everyone else.
The use of this term is not common at all in poker anymore, mostly replaced by the words "Donkey," "Donk,", and "Fish."
Pigeon used in a sentence:
"Wow, Phill Hellmuth is such a pigeon, he completely butchered that hand!"