When we say "no big thing," we mean that something is not a big deal or not significant. There are various synonyms that can be used instead of this casual phrase. For example, we can say "no problem," "it's alright," "nothing to fret about," "nothing to worry about," "not a concern," "not a fuss," "no sweat," or "nothing of consequence." These alternatives can be used based on the situation and the tone required. Overall, using synonyms can add variety and depth to our conversations while conveying the same meaning as "no big thing".