Supposer is a word that means to assume or believe something without concrete evidence, and there are a variety of synonyms that can be used instead. One could use the word surmise, which means to suppose or suspect something without proof. Another synonym for supposer is conjecture, which means to form an opinion or conclusion through speculation. One could also use the word speculate, which means to consider or think about something without being sure of its truthfulness. Other synonyms for supposer could include presume, postulate, or presuppose. Ultimately, there are many words that can be used instead of supposer to convey the idea of making an assumption or guess.