To go behind someones back and betray them

To enviously kill a friend or someone who trusted you unbeknownst to them

When the location of an unsuspecting person is acquired, typically through their “friends” or a girl they might know. This location is usually given to the opps in order for this unsuspecting person to be set up, robbed, and sometimes killed. Lil Durk on his feature for “Top” says “Your close homie’ll try to Backdoor you if you let him”

When someone tryna take ya girl on the low

Being a middle man to a drug transaction. Short stopping a play. Taking out of the money or drugs that someone else trusted you with.

In League Of Legends , it means you siege unguarded enemy turrets while your team is fighting the other team elsewhere.