ETYMOLOGY
from slap (to lap → Eng. dial.) + sauce
EXAMPLE
“…At dinner and supper the table doth craue
good fellowly neighbour good manner to haue.
Aduise thee well therefore, ere tongue be too free,
or slapsauce be noted too saucie to bee…”
From: Points of Huswifrie
In Fiue Hundreth Points of Good Husbandry
By Thomas Tusser, 1573
Lexicophilia 