A term used to describe an ugly gay male, originally at the beach, but now anywhere, and is used by gay males. It originates from the term Poseidon, one of the God's from Greek mythology. A person can only be a poseido if they are sweaty or greasy, but being both is acceptable.
It can also be a verb used by men who hook up with a poseido. Either you've been poseidoed, poseido'd, pozeido'ed, etc. If you believe yourself to be greasy you can be a poseido as well.
Daintey: "Haha you've been poseido'd!"
Locien: "Shut up dude, I was drunk!"
Wyrmy: "I'm totally a poseido, but I hooked up with Locien."