The noun sop means:
– literally: a piece of bread dipped in gravy, soup or sauce;
– figuratively: a thing of no great value given or done as a concession to appease someone whose main concerns or demands are not being met.

This figurative use alludes to the following passage from the Aeneid, by the Roman poet Virgil (Publius Vergilius Maro – 70-19 BC), in which the Sybil, who guides Aeneas through the underworld, throws a specially drugged cake to Cerberus, the three-headed watchdog which guards the entrance:

     (translated by A. S. Kline – Poetry in Translation):
Huge Cerberus sets these regions echoing with his triple-throated
howling, crouching monstrously in a cave opposite.
Seeing the snakes rearing round his neck, the prophetess
threw him a pellet, a soporific of honey and drugged wheat.
Opening his three throats, in rabid hunger, he seized
what she threw and, flexing his massive spine, sank to earth
spreading his giant bulk over the whole cave-floor.
     Latin text from: Bucolics, Aeneid, and Georgics of Vergil – J. B. Greenough (Boston, 1900):
Cerberus haec ingens latratu regna trifauci
personat, adverso recubans immanis in antro.
Cui vates, horrere videns iam colla colubris,
melle soporatam et medicatis frugibus offam
obicit. Ille fame rabida tria guttura pandens
corripit obiectam, atque immania terga resolvit
fusus humi, totoque ingens extenditur antro.

In The works of Virgil containing his Pastorals, Georgics and Aeneis (London, 1697), by the English poet, literary critic, translator and playwright John Dryden (1631-1700), the Sibyl throws “a Sop, in Honey steep’d, […] mix’d with pow’rful Drugs”:

No sooner landed, in his Den they found
The triple Porter of the Stygian Sound:
Grim Cerberus; who soon began to rear
His crested Snakes, and arm’d his bristling Hair.
The prudent Sibyl had before prepar’d
A Sop, in Honey steep’d, to charm the Guard.
Which, mix’d with pow’rful Drugs, she cast before
His greedy grinning Jaws, just op’d to roar:
With three enormous Mouths he gapes; and streight,
With Hunger prest, devours the pleasing Bait.
Long draughts of Sleep his monstrous Limbs enslave;
He reels, and falling, fills the spacious Cave.

The earliest figurative use of sop that I have found is from the following dialogue between a bailiff and Colonel Blunt in The Committee, a comedy (London, 1692), by the English playwright and politician Robert Howard (1626-1698):

                              Colonel Blunt brought in by Bayliffs.
– 1 B. I, I, we thought how well you’d get Bail.
– C. Bl. Why, you unconscionable Rascal, are you
Angry that I am unlucky, or do you want some Fees?
I’ll perish in a dungeon before I’ll consume with throwing
Sops to such Curs.

The phrase to give a sop to Cerberus, meaning to offer something to placate or propitiate somebody, is first recorded in Love for Love (London, 1695), a comedy by the English playwright and poet William Congreve (1670-1729); at the beginning of the play, Trapland, “a Scrivener”, is calling on Valentine to collect “a Debt […] of 1500 l. of pretty long standing”, and Valentine says to his friend Scandal:

Bid Trapland come in. If I can give that Cerberus a Sop, I shall be at rest for one day.