贻笑大方

idiom

to make a fool of oneself in front of experts; to become a laughingstock among connoisseurs; to invite ridicule from those who know better

Originally '大方' referred to '大方之家' (masters of great learning). This idiom describes someone who displays ignorance or poor skill before true experts, exposing themselves to justified ridicule. Often used as a warning or self-deprecating admission.