Why do we say 'gonna', and how does a walking verb become grammar?

'Gonna' is the latest stage of a story that began with a walk. The going-to future originated in the late 15th century, when the spatial sense of the verb go was extended to a temporal one, the same kind of change seen in the preposition before. The original construction involved physical movement with an intention, as in 'I am going outside to harvest the crop.' Then the location became unnecessary, and the expression was reinterpreted to represent a near future.

One of the early examples comes from a 1483 publication about Adam of Eynsham, a medieval English chronicler: a soul that 'was goyng to be broughte into helle'. The soul is not going anywhere on foot; a verb of motion is being used for the future.

Linguists call this grammaticalization: words changing from representing objects or actions to serving grammatical functions. The English verb will did the same thing: Old English willan meant 'to want' or 'to wish', and it became the auxiliary that expresses intention or simply futurity. Today it shrinks to 'll, a clitic that cannot take stress.

The two senses really are separate now. You can say 'I'm going to go to the store now', using the future going to and the walking go side by side. And the contraction 'gonna' is available only for the future meaning: 'I'm gonna swim' explicitly means futurity, not a trip to the pool.