What does 계간 (鷄姦) literally mean?

 ANSWER: chicken rape

The Sino-Korean word 계간 (鷄姦) translates a "sodomy," but it literally means "chicken (鷄) adultery (姦)" or "chicken rape." 

So, why does "chicken rape" mean "sodomy"? Because a female chicken has only an anus and no separate sexual orifice.

From Dong-A's Prime Korean-English Dictionary