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."
To split the purchasing of a pizza equally among two parties.
Since its inception, the term "halfsies on a" has extended past usage in pizza realted terms and can be used in ordinary conversation pertaining to more typical purchases.
The term's first documented usage was by two white suburban Detroiters during Colombus Day weekend 2004.
"I'm famished, you wanna throw halfsies on a pie?"