Pronunciation: [sˌiːðəlˈɛs] (IPA)
Correct spelling for the English word "seatheless" is [sˌiːðəlˈɛs], [sˌiːðəlˈɛs], [s_ˌiː_ð_ə_l_ˈɛ_s] (IPA phonetic alphabet).
Correct spelling for seatheless