When you can't decide between two things, so you and another person (usually a friend or acquaintance) each purchase one of the desired items and you both exchange half of each item with one another.
Note: Most often done when ordering food. This way you can eat both items you want instead of ordering two meals and looking like a fat ass.
Person 1: "Man, I can't decide between the teriyaki chicken or the club."
Person 2: "Yeah, me neither."
Person 1: "Double halfsies?"
Person 2: "You read my mind."
This is when you use a high ball glass and pour half the cup full of vodka and then the rest add flavored mineral water. It is customary when this is done for first-timers to show the person so they see that half of their drink is vodka.
Bob: Yo, what are we drinking tonight?
Gerald: We sipping on a halfsee.
so when you help someone with their internal assignment you don't get the full credit for it but you don't get no credit for it so you halfly get credit for it.