鞭长莫及

biāncháng-mòjí

(1) [although the whip is long, it does not reach the horse's belly--cannot do it much as one would like do] 本意为马鞭虽长, 但打不到马肚子上。 喻虽有力, 力量亦达不到

虽鞭之长, 不及马腹。 --《左传·宣公十五年》

(2) 后用以喻力不能及。 亦作"鞭不及腹"