wú appears chiefly in
鼯鼠 (flying squirrel - family Pteromyini), nocturnal squirrels with a patagium (skin flap) between fore and hind limbs that lets them glide between trees. The classical chengyu
鼯鼠之技 ('flying-squirrel's skills', from 《
荀子》) describes someone with five mediocre skills and no real mastery: the flying squirrel can climb, swim, dig, run, and glide - but does none of them well. A polite put-down still used in formal writing.