Webster's Revised Unabridged Dictionary · 1913

gonna

gonna (gun"nu) , phrase

[phonetic transcription of colloquial speech.]

Going to; as, who's gonna get the milk?. [colloquial, phonetic spelling]

Phonetic transcriptions of rapid colloquial speech are sometimes used in place of the grammatically proper spelling in order to provide a flavor of the original spirit of a spoken dialogue. See also the related entries for gotta, wanna, and wannabe.