The word "unimpeachably" is an adverb that means something is done in a way that cannot be questioned or doubted. It suggests that something is completely reliable, trustworthy, or of very high quality.
In more formal or academic writing, "unimpeachably" can be used to emphasize the integrity or credibility of a person or argument.
While "unimpeachably" refers specifically to being beyond question, "impeach" can also relate to legal procedures, especially in politics, where public officials can be charged with misconduct.
Currently, there are no specific idioms or phrasal verbs that contain "unimpeachably," but you might find phrases like "beyond a shadow of a doubt" or "beyond reproach" convey similar meanings.
In summary, "unimpeachably" is a strong adverb used to affirm the reliability or quality of something, making it clear that there is no doubt or criticism possible.