Cerberus, in classical mythology, was the three-headed dog that guarded the entrance to Hades, permitting the dead to enter, but preventing their shades from leaving.
Sometimes those who entered were greeted in friendly manner by the several heads, but others might be met by fierce and angry snarls.
Accordingly, friends placed in the hands of those who died honey cakes to be fed to Cerberus, thus to permit them a friendly entrance, as described in Bishop Gavin Douglas’s translation of the Aeneid (1513), VI, vi, 69:
Cerberus, the hiduus hund, that regioun
Fordynnys, barkand with thre mowthis fowm,
Onmefurabill in his cave quher he lay
Richt our for gane thame in the hie way:
Quhom till the prophete, behaldand quhou in hy
Hys nekkis wolx of eddyrris all gryfly
A fop, ftepyt intill hunny als faft,
And of enchant it cornys maid, gan caft.
Hence, figuratively, “a sop to Cerberus” is any gift or compliment or the like that may placate an opponent, allay suspicion, or distract vigilance.