1.To have sex without a sound
2. To have sex without the other person knowing

the whale tail is the shape formed when a g-string rides up high over a womans trousers

To have sex in a quick and intense manner.

To penetrate another person without that person having knowledge or noticing.

To have sex in a bunk bed without your bunkmate noticing.

If I heard this right, it refers to the action of being sexed half-willing, whether out of obligation or whatever motivation (this isn't referring to rape, mind you). It comes from a movie called Dog Park, and I'd like to see the term used more.

When you ( a male) is having sex with two other people ( male or female) and you fuck them in the butts back to back. Hence, you see an ASS, you see an ASS, and you went IN, or assassin ( which is a word used to describe a ninja).