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1.
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?"

"Let's go halsies on a prostie."
by Mangelo January 23, 2006