A very small penis, good for learning with but useless after that. Generally small in length and girth so it is less painful for virgins, but once they get used to having sex it is of an insufficient size. The term is derived from skiing and snowboarding where the starter hill for beginners, or bunny hill, is shorter and less steep and therefore easier for those beginners to cope with. Although the bunny hill is generally regarded as boring for those who know how to ski or snowboard, or in this case have sex. Often anything less than five inches long (12cm) and 4 and half inches in girth (11cm), but is definitely relative to the dimensions of the other sexual partner.
I was lucky, my first time was with a Bunny Hill dick so it didn't hurt too much, of course I'm looking for a real dick now LOL