Correct spelling for the English word "haddest sex" is [hˈadəst sˈɛks], [hˈadəst sˈɛks], [h_ˈa_d_ə_s_t s_ˈɛ_k_s] (IPA phonetic alphabet).
The phrase "haddest sex" does not have an established etymology as it is not a recognized word or phrase in standard English. It is important to note that "haddest" is an archaic second-person singular past tense verb form of "to have", and "sex" refers to a sexual activity or intercourse. However, it is unclear how or why these words have been combined in this specific manner.