When someone does something accidentally or unintentionally, it is often described as being done "not on purpose". However, there are numerous synonyms that can be used to express the same meaning. Some alternatives include "unintentionally", "accidentally", "inadvertently", "without intention", "unwittingly", "unconsciously", "by mistake", "by chance", "incidentally" and "serendipitously". These words can be used to express a variety of situations, from minor mistakes to significant events that happened without any planning or intention. When trying to convey the idea that something occurred without purpose, consider using one of these synonyms to add depth and nuance to your language.