Usually used in the same sentence as booty/ass/butt/etc....
Refers to a very wide ass that seems to almost grab hold of both sides of somebody grinding on that ass's hips.
Franco: WOAH! So her ass was so big it was almost as if she was picking you up and down with it? Like an octopus would with it's tentacles!
Renzo: Precisely! Her ass was TENTACTEROUS!