A book written for a mature audience. This type of book usually has sex scenes and other content that would be inappropriate for younger readers. This is a term used to cover up the fact you are reading a "naughty book".

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