Bribes, or rather money for bribes, fees, etc.
“In Ireland the machinery of a political movement will not work unless there is plenty of palm-oil to prevent friction.” —Irish Seditions from 1792 to 1880, p. 39.
“The rich may escape with whole skins, but those without `palm-oil' have scant mercy.” —Nineteenth Century, Aug., 1892, p. 312.
Palm Sunday The Sunday next before Easter. So called in memory of Christ's triumphant entry into Jerusalem, when the multitude strewed the way with palm branches and leaves. (John xii.)
Whose banks received the blood of many thousand men, On `Sad Palm Sunday' slain, that Towton field we call ... The bloodiest field betwixt the White Rose and the Red.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894