Ukemi = bottom: the verb ukeru means to "take, receive, accept," thus uke (receiving) mi (body, party) means "one who receives," or, "a bottom." Neko = bottom: neko literally means "cat". It has exactly the same meaning as ukemi, but in a slangier way.
[OC] Nothing Describes Me More Than Wearing A Shirt Covered In Cat Hair And Showing My Tits In Terrible Bathroom Light.