The Boothole is at the end of a cigar/cigarette/blunt etc. It is the end that you put your mouth to and suck the smoke out of. Sometimes, the tobacco/weed etc. inside will come out, representing shit. This is called the boothole! Now you have something to call that hole that you never knew what to call!
Collin: "Dude, what's wrong?"
Cody: "Oh, nothing. A little tobacco just got in my mouth!"
Collin: "Oh, did it fall out the boothole!"
Cody: "Yes, unfortunately..."