What you say in the grocery store when you reach the end of the frozen foods aisle.
"They're outta cool whip. I gotta go. I just shot my wad."
Means "I'm done, that's all I have the energy/money,willingness/desire for, there is no more I have to say/add/do."
Importer: "I'm sorry the cap on your bottle of Pickapeppa Sauce was defective. Maybe the distributor can arrange for a replacement."
Consumer: "Thanks but I've shot my wad on this subject." (I'm done, don't want to take it further.)
PS One hopes the provenance of this perhaps somewhat dated colloquialism was not an ugly sexual allusion to the male orgasm (back in the 50's when scumbag was an ugly slang term for condom) but at the time if it was, it did not seem to be generally thought of as such. But looking back, I dunno.
"dude! we need to find some hookers"; na, I met someone earlyer and shot my wad.
It actually comes from the old days of musket guns, where a wad (strips of cloth) was packed down in the barrel to create a seal between the gunpowder and the bullet, increasing the internal pressure when the gunpowder fired, and thus speeding the bullet.
A musket is a muzzle-loaded, smoothbore firearm, fired from the shoulder. Muskets were designed for use by infantry.
A soldier armed with a musket had the designation "musketman" or "musketeer".
"I ran out of ammo, General Washington, so I shot my wad."
(When soldiers ran out of bullets, they would resort to "shooting their wad".)