The area between your legs. Your crotch.

If you are a little pudgy, your gut may sag down when doing push ups or copulating. This can be particularly unattractive as during vigorous sex, it may swing from side to side, back and forth, much like a metronome. In extreme cases, the folds of fat, or female breasts, may slap together making a sound not dissimilar to the sound of a sealion performing for food at shows at many zoos. This noise is known as flap or tat slapping.

Very simple. This is your pet's fucking stomach.