A vehicle with no engine that is pulled by another vehicle, or a short film that advertises a longer movie or show.
It comes from 'trail', meaning 'to pull' or 'drag', plus the suffix '-er' meaning 'thing that'. The movie sense began because these ads were originally shown after (trailing) the main film.
Movie trailers used to come at the end of a showing, but people kept leaving early—so theaters moved them to the front and the name stayed. It’s a nice reminder that words can keep telling old stories even after habits change.
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