'Gonna' shows how a verb of motion turned into a grammatical marker for the future

'Going to' once meant only physical movement toward a destination, as in 'I am going to the store.' Through a process linguists call grammaticalization, it lost that literal sense of motion and became a marker of future intention even with no travel involved, as in 'it's going to rain,' with 'gonna' as the phonologically worn-down result.

— Paul J. Hopper & Elizabeth Closs Traugott, Grammaticalization — Cambridge University Press, 2nd edition, 2003
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