To grease someone's palm; literally to dosh them up. From pisto, chapin slang for dosh.
The most recent accounts I have heard about the death of our friend in Guatemala last weekend are taking the regretably rather everyday story of the accidental death of a motorcyclist and turning it into a proper chapin folk-tale.
The woman that got down from the car to kick him (having just run him over) turns out to have been a judge. There were other people in the car and they promptly joined in. The judge has no doubt since had to pistear the local cops, as our friend was one of many hundreds of bikers on the road that morning in Chiquimula, and several of his comrades on the Caravana del Zorro took photos of the incident.