A vulgar term that everyone uses. An intensifier, usually negative, as in, the phrase popular with drivers, "pinche hijo de La Chingada," "lousy son of the archetypal whore." But it's occasionally used in a positive sense, as in "que pinche culota!" "what a piece of ass!"
A Mexican writer who had just come back from Spain told me "I don't understand how they survive without 'pinche'.