WHo was the first polygamist in the Bible?

Lamech. (There are two different Lamechs; the one descended from Cain rather than Seth is the polygamist.) While the narrative names Lamech's two wives it does not explicitly state that he was the first polygamist... the genealogy in Genesis only names a few people specifically, and simply refers generically to other sons and daughters besides the ones mentioned by name. Lamech is just the first polygamist that's unambiguously pointed out; there may or may not have been others before him.