1. Telling your bro to suck it up do whatever it is he's supposed to do
2. Retrieving an alcoholic beverage
3. Picking up girls at a party
It is generally kept in its perfect form for the sake of preserving its perfection however any slight variations are allowed to suit the situation. People using the term should be aware of the plural form Grab Ds which can sound like Grab Deez or more to the point Deez Nuts. Which means something different. Use wisely.
Dude #2: Yeah bro, Grab D
Dude #1: Dude my girlfriend really broke my heart this time.
Dude #2: WTF man, stop fucking crying like a little bitch and Grab D.
Dude #3: Yeah man we're hitting Tridelts party tonight you can just Grab D all night there.