A person who rides on something like a horse, bicycle, or motorcycle; also a clause or condition added to a document.
From Old English 'ridere,' from 'ride' (to sit and travel on), plus the agent suffix '-er.' The word has meant a person on horseback since Anglo-Saxon times. The meaning of an added clause came later, from the idea of something that 'rides on' or attaches to an existing document.
Congress uses the 'rider' trick all the time: a politician will sneak a totally unrelated law into a bill by attaching it as a rider, so if the main bill passes, their rider passes too—it's a legal loophole that's been around for centuries!
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