It means, "how much do you have remaining?"
If you're waiting on someone to do something, you may ask them this to get an idea of how much longer it will take for them to complete their work.
In the Southern United States, the word "like" will be used in place of "lack"; see: "How much you like?"
John: I can't come over until I've finished painting my fence.
Mary: How much you lack?
John: I'm nearly done. I have about 30 minutes left.
Said on being introduced to someone
This is my wife, Pleased to meet you
rhetorical question designed to highlight the subject's idiocy, to the point that it's a marvel they are able to even do basic things like dress themselves
person 1: doesn't chocolate milk come from brown cows?
person 2: how do you dress yourself in the morning?
A phrase said when two people are in an altercation and one of the two moves uncomfortably close to the other, and causes the other to shout, "Why you in my phisical?".
Douche: You hittin' on my girl, bro!?
Nerd: Dude, why you getting so close? Why you in my phisical? Get outta my phisical! I said get the fuck out of my shisical, motherfucker!!!
*Karate kicks douche in face, breaking jaw*
Famous words uttered by John Jones
John Jones: Wait. Wait wait wait. Can you hear me?
Describes the action of a very happy person.
Also part of a song.
If you're happy and you know it clap your hands
*clap* *clap*