A simple procedure execute by curious aliens or doctors to examine the lower rectum and other internal organs of a person/animal. They (gently) insert a tool, tentacle or finger into your anus. This allows them to feel your interior and explore it.
Kang: My name is Kang, and this is my sister, Kodos.
Kodos: Hello.
Homer: I suppose you wanna probe me. Well, might as well get it over with.
Kang: Stop! We have reached the limits of what rectal probing can teach us.
(The Simpsons - S8E1: Treehouse of Horror VII)
Someone who takes a strong liking to inserting randoms objects up peoples assholes to probe areas of the intestine and colon not yet studied by doctors and air port security guards.
When I asked Kristen what she was going to get her degree in at Auburn, she told me butt probing. When I asked her why she said it was because she liked taking householdobjects and ramming them into people's assholes.