poka
If you don't count surgery, probably reaching up a cows rectum.
It is called an enema.
Rectum? Probably not; the rectum is inside the body, and you probably couldn't get to it. The anus? The anus is the ring of muscle that is the exit from the rectum. If he/she maintains some basic hygiene and cleanliness, and isn't ill with ebola or cholera or dysentery, it's probably safe to do. Kinky, perhaps, but safe.
The one your poop comes out of. But it is called an anus, not a rectum. The rectum is up inside you.
up your rectum
Put a condom on your tongue on cut it open and put the cut up condom over the rectum. You do not want fecal bacteria in your mouth.
A frog's rectum takes bacon and poops it on you and then hops up your butt and strangles your 2345678.
Colonoscopy procedures look at the areas of the rectum and large intestine, as well as the colon. They put a scope up inside you so they can see what they are doing up there.
Up my rectum
The meniscus
When you place a can of 'Spam' up your dogs rectum.
it is up your rectum