in heat
When a creature (most likely an amimal, but can apply to humans) is so horny, it'll hump anyhthing in sight.
Example:
Mona: OMG! Look at Jack!
Mae: What the fuck is he doing?
Jack(pulling out of the dog's ass): Ooooh, yeah! Thanks again, sparky!
Sparky: Woof!
Mona: OMG! Look at Jack!
Mae: What the fuck is he doing?
Jack(pulling out of the dog's ass): Ooooh, yeah! Thanks again, sparky!
Sparky: Woof!