Any joint rolled by a certain Roy Ferari. This Roy rolls really crusty joints, and so the name 'Custy Roy Joint' came into play whenever he rolled a joint. A 'Croint' is a variation of that phrase, utilising the first letters of Custy and Roy and placing them before the word joint with the J removed. Croints have a certain resemblence and definite traits, and so thereforre when somebody other than Roy Ferrari rolls a custy looking joint, it is reffered to as a croint. It is an insult to the person's rolling ability.
Bob: "Hey man, jets go burn this jay I just spun."
Randy: "Aw, Eww, man, I aint smoking that custy croint!"
John: "Let's roll a joint guys."
Roy Ferari: "I'll roll it"
Everyone in the room: "Fuck no! We aint smoking no croints!"