It Means to contribute $5.00 to someone who has some good ass weed so they can smoke it too. Usually when one cops the weed they make others who want in put some money down to let them smoke it as well.

Also a song by Luniz a Bay Area rapper "I got 5 on it"
"no, me ain't my homie to begin with,
it's too many heads to be poppin at my friend hit it
unless you pull out the phat, crispy
5 dollar bill on the real before its history
cos fools be havin the vaccum lungs,
an if you let em hit it for free,
you hellar "dum-dum-dum-dum"

I got 5 on it,
grab your 40, let's get keyed
I got 5 on it,
messin wit that Indo weed
I got 5 on it,
it's got me stuck and not go back
I got 5 on it,
potna lets go half on a sack

- Luniz
by yayman October 19, 2007
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