Today it means: A bribe or extorted money, usually relatively small in amount, provided to a low-level government official or business person, in order to expedite a business decision, shipment, or other transaction, especially in a country where such payments are not unusual. Full definition →
By a metaphor in which facilitating human socioeconomic interaction is portrayed as lubricating machinery (e.g., oiling the gears, greasing the wheels); in this regard, compare also social lubricant.
Etymology from Wiktionary, CC BY-SA 4.0.