When a penis (substitution for a penis, ie: cucumber, dildo... hamster) is quickly placed into and out of the anus no further than the tip. Meant to sensualize
If you're a man and wants to experiment with dudes, without being considered a homosexual, just ask for a rimjob.
Someone insignificant who doesn't deserve recognition due to their lousy actions, they are not even worth calling an arsehole; just a slight tear in that arsehole - hence rimrip.