know one's stuff

adequate capable decent efficient proficient qualified skilled

adapted all around appropriate au fait being a pistol clever complete crisp dynamite endowed enough equal fireball fit fool good know ins and outs know one's business know the answers know the ropes know the score no slouch on the ball paid one's dues pertinent polished satisfactory savvy sufficient suitable there up to it up to snuff up to speed wicked

impotent incapable incompetent inept insufficient unable unqualified unskilled unsuitable

inadequate inefficient

know backwards and forwards know by rote know inside out know like a book