Prime slap-up or slap-bang up. Very exquisite or dashing. Here slap is a playful synonym of dashing, and “up” is the Latin super, as in “superfine.” The dress of a dandy or the equipage of an exquisite is “slap-up,” “prime slap-up,” or “slap-bang-up.”
“[The] more slap-up still have the shields painted on the panels with the coronet over.” —Thackeray.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894