The correct spelling of the word is "doubtful". Though it may sound like "doubtous" /daʊtəs/, the correct spelling uses the letter "f" instead of "ous". Phonetically, the word is transcribed as /ˈdaʊtfəl/. It means uncertain, hesitant, or not completely convinced. The spelling of this word is essential for effective communication and understanding in both written and verbal communication, and it is important to always use the correct spelling in formal written communication.
Doubtful.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.