The rear end of an animal whose front end is underground.
The butt of an animal
Rear end and butt are essentially the same thing.
exists s1 s2 s3 x1 x2.(a_topic(x1) & of(x1,x2) & Theme(s3,x1) & a_underground(s3) & n_end(x1) & n_end(x1) & a_front(s2) & Theme(s2,x1) & of(x1,x2) & n_animal(x2) & a_rear(s1) & Theme(s1,x1))
exists x1 x2.(n_butt(x1) & a_topic(x1) & of(x1,x2) & n_animal(x2))