The earliest such example I have (thanks to my brother Glen) is "F--- tha Police" by N.W.A., from 1988: "I'ma kick your ass." There's also House of Pain's "I'm a Swing It" from 1994, with the line "Ya dis me and I'm a dis ya back / I'm a swing it." Imma continued to be used in more and more songs, though not in hit singles. More @Wikipedia
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