(US, slang) A bottle containing 40 fluid ounces of malt liquor beer.
But when it's all said and done, I'll be forty / Before I know it with a 40 on the porch tellin' stories 2000, “Drug Ballad”, in The Marshall Mathers LP, performed by Eminem:
[Chorus:Michael Marshall] I got 5 on it (got it, good), grab your 40 let’s get keyed. I got 5 on it, messin’ with that Indo weed. 1995 July 4, “I got 5 on it”, in Operation Stackola, performed by Luniz and Michael Marshall (singer):