The Montana Miscarriage is a compromise for your prude wife to finally give into anal sex "what she doesn't want to do", and in return you get her pregnant, "which you don't want to do. The miscarriage part comes when you reneg on the deal by not pulling out and jizzing in the vagina, but instead, staying the course and dumping the children into the butthole.

A jack off buddy (from 'cum' /'come', meaning 'semen', and 'companion'); a friend that you get together with for sessions of solo or mutual masturbation
Variant of: cumpanion (from 'companion' & 'cum'); jack off buddy & wank bud & JO buddy
Similar to: fuck buddy