This is a colloquial phrase that literally translates as 'to smear one's hand'. It is usually used to imply bribery, that is, giving someone money or gifts to influence their actions or decisions in your favor.
Examples
- Spanish: El político corrupto siempre espera que le unten la mano para hacer favores.
- English: The corrupt politician always expects his hand to be greased in order to do favors.