To defy attempts at description; to be difficult or impossible to describe or explain. My mother loves decorating for Christmas, so during the holidays, her house is fit for Santa himself. It truly beggars belief. My boss's incompetence is astounding. It truly beggars all belief.
beggar (all) description
to defy description; to be unable to be described. The house was a horrible mess. The place beggared description. Our reaction to the proposal beggars description. We were deeply disturbed for days.
Defy or outdo any possible description, as in The stage set was so elaborate, it beggared description. This term, alluding to the idea that words are insufficient to do something justice, was already used by Shakespeare in Antony and Cleopatra (2:2), "For her own person It beggared all description."