Jeans with a hammer loop. Originally intended for carpenters, enabling them to work in a more efficient manner.
The term 'jarpenters' only applies to jeans in which the loop is not actually intended to hold a hammer and might therefore be decorated with a brand logo.
Guy 1: "See that dude at the end of the bar wearing those Tommy jarpenters? I bet we could score some hash off of him. I'll go ask him...if that visible boner he's got ever goes away. What a sick bastard."
Guy 2: "He should tuck that thing in his hammer loop. Put those jarpenters to use."