Comical, melodramatically distressed exclamation uttered by stereotypically snooty actors after being slighted in what they perceive to be an abhorrent manner. Often preceded, in the middle of the fifteenth take, by a statement such as "I simply cannot work like this!"
The "trailer" mentioned in this phrase does not refer to a white trash trailer park kind of trailer or a movie trailer, either, but rather the actors' trailer where actors get ready (via makeup, costumes, etc.) to shoot a scene.
Since this phrase and its connotations (e.g. the aforementioned snootiness of the speaker) have been pretty well assimilated into the collective consciousness, or national psyche, or whatever you call it, it has become popular to drop it into a set of out-takes to achieve instant funny similar to that generated by monkeys or pants.