Boneheadedly is an adverb that is used to describe someone's actions or decisions as foolish or stupid. If you are looking for synonyms for this word, you can consider using words like obtusely, dimly, stupidly, ignorantly, or senselessly instead. These words give a similar meaning to boneheadedly and can be used interchangeably depending on the context. Other synonyms for boneheadedly include foolishly, brainlessly, thickly, and blockheadedly. All these words essentially convey the same idea of a person acting in a foolish or unintelligent manner. So, if you want to add some variety to your writing, consider using these synonyms for boneheadedly.