two major definitions:
1.) laying down for weed, usually this involves paying $5 out of a Dub sack (20$) but can also be the cost to drop on half of a blunt, or to buy half a dime.
2.) calling 5 minute rule on something.
1.) buying tonight? damn, i got 5 on it then.
2.) don't even try it bitch... see that chair over there? i already got 5 on it.