A taxi driver or someone who drives a hack (an old, cheap taxi or carriage for hire).
From hack (a taxi or carriage for public hire, from the sense of 'hackney') + driver. 'Hackney' originally referred to a type of horse breed used for general riding, then became the word for public carriages, and eventually applied to taxis.
Hackney horses gave their name to hackney carriages, which then gave their name to hackers and hack drivers—all because they were ordinary, workaday animals you could hire for routine transport, nothing fancy!
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