Lewis & Short

ălăpa, ae, f. [akin to -cello, to smite, as if calapa; cf. κόλαφος], a stroke or blow upon the cheek with the open hand, a box on the ear: ducere gravem alapam alicui, to give, Phaedr. 5, 3: ministri eum alipis caedebant, Vulg. Marc. 14, 65; ib. Joan. 18, 22; 19, 3; esp. among actors, for the purpose of exciting a laugh among their auditors, * Juv. 8, 192; * Mart. 5, 61, 11.
When a slave was emancipated, his master gave him an alapa; hence, poet.: multo majoris alapae mecum veneunt, i. e. with me freedom is much more dearly purchased, Phaedr. 2, 5, 25.