鞭长莫及（鞭長莫及）　

◎ 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”