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There is also an element of "hard work" in the term "grind" (imagine grinding something- can mean the repetitive work of moving in circles around a millstone, or the act of being crushed into powder. All imagery suggesting hard work). To answer your question, it possibly means being "stuck in traffic is like being in a repetitive, crushing situation that lasts from 9 am to 5pm".

9 to 5 grind = working a job that's 9am - 5pm. Kind of standard time. Maybe the person got off work and is stuck in busy traffic