I've adde a little to it. Slap up. To have a slap up meal means to eat well. The expression goes back to the time of Charles Dickens, when it was a "slap-bang" meal, derived from cheap eating houses, where one slapped one's money down as the food was banged on the table.

What does a slap up mean?

adjective British Informal. excellent; first-rate: a slap-up do.

What does the slang slapper mean?

/ (ˈslæpə) / noun. British slang a promiscuous woman.