The word "impeachable" means that someone's actions or behavior is considered serious enough to be questioned and possibly led to their removal from a position of power, such as a government official or president. Full definition
That is, the claim here is that Federalist # 65 is open to a broader sort of impeachable offense than specific statutory crimes or violations of the Constitution. (firstthings.com)
Members of the House of Representatives on Tuesday said President Muhammadu Buhari committed an «impeachable offence» by authorising the purchase of $ 462m jets for the military without appropriation by the National Assembly. (nigeriapoliticsonline.com)
But a mentally unstable president isn't impeachable. (timesunion.com)