A sop in the pan. A bonnebouche, tit-bit, dainty morsel; a piece of bread soaked in the dripping of meat caught in a dripping-pan; also a bribe. (See below.)
To give a sop to Cerberus. To give a bribe, to quiet a troublesome customer. Cerberus is Pluto's three-headed dog, stationed at the gates of the infernal regions. When persons died the Greeks and Romans used to put a cake in their hands as a sop to Cerberus, to allow them to pass without molestation.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894