When you buy a large amount of weed or other drug and intend to sell it in smaller deal's, then you get 'personal' weed that yo' can smoke for free. :-)
"When I buy a pound I usually take out an 'oscar' for personal smoke."
A personal is used to describe an entire unit of marijuana to be smoked by one person. It is most often used at a party or group gathering when one shows up late and needs to catch up to everyone else's level of high-ness.
Casper Milky Toes: "Shit, did you just now get off work?"
Scruff Mcgruff: "Yeah, it was a nightmare."
Casper Milky Toes: "Sit down, relax man. I'm gonna pack you up a personal."