A person who rides a dicycle or bicycle, or someone who specializes in studying dicyclic structures in chemistry or botany.
From 'dicycle' + '-ist' (one who practices or specializes in). The '-ist' suffix creates agent nouns, identifying a person by their activity or expertise.
The '-ist' suffix is a job-title factory—combine it with almost any noun and you've created a new profession, which is how 'cyclist' became 'dicyclist'!
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