Correct spelling for the English word "had do" is [hɐd dˈuː], [hɐd dˈuː], [h_ɐ_d d_ˈuː] (IPA phonetic alphabet).
The etymology of the phrase had do is derived from Old English. It is a combination of the past tense of the verb have (had) and the word do. In Old English, the phrase had do originally meant to have to do or to be obliged to do. Over time, it evolved into the modern expression had to do.