Why is the ninth month called September?
1890, "vehicle pulled by another," agent noun from trail (v.); originally a small carriage drawn along by a bicycle. Meaning "preview of a coming movie" first attested 1928. Trailer park recorded by 1936. Trailer trash in use by 1986.
A preview of a coming movie, a brief travelogue, or another short film shown before or after a feature movie (1928+ Movie studio)