Slapper \Slap"per\, n.1. One who, or that which, slaps.
2. Anything monstrous; a whopper. [Slang] --Grose.
Source: Webster's Revised Unabridged Dictionary (1913)
Slapper \Slap"per\, Slapping \Slap"ping\, a. Very large; monstrous; big. [Slang.]