Phrase used to express enthusiastic endorsement of an idea, particularly if the idea pertains to the recent or imminent acquisition of goods or information. While some believe the term descends from "sop it up with a biscuit" or the practice of slapping someone upside the head, "slap it up" most likely refers to the literal action of placing a desired object on a surface with considerable force, as a means of presentational flourish.
"Governor, may I offer you a glass of eighteen-year-old single malt?"
"Yeah bitch, slap it up. On the rocks"
"Guys, guys...I got the promotion!"
"Right on Steve, way to slap it up. Let's go celebrate."