A term used to describe any guy that has fornicated with the same woman as you. It is like half brothers, but closer.
Mike: Hey bro, I just smashed with Shaquanda last night.
Garrett: Dude, I just beat those guts last week! We are shaft brothers. You are like my family now!
Two guys that are in a bromance that repress their sexual energy for each other.
Aran and James are such shaft brothers! They should just fuck already!