弁慶の泣き所
(Benkei no nakidokoro; “The place that makes Benkei weep”)

Definition:

The shins; someone’s weak point, an Achilles’ Heel. The term 泣き所 by itself can mean a shin or a weak point; this phrase intensifies the image by picturing a vital point so vital that even a hardened warrior will cry if struck there. A strong person’s only weakness.

Breakdown:

弁慶 is a proper noun, the title of our old friend, the warrior-monk Benkei. It is connected by the associative particle の (no) to the noun phrase 泣き所 (nakidokoro), which comprises the noun 所 (tokoro), “place,” and the conjunctive form of the verb 泣く (naku), “to cry,” “to weep.”

Notes:

This appears to be a surprisingly recent addition to the Japanese language, attested (at least in its meaning of “one’s weakest point”) only since the early 1900s.

Example sentence:

「先輩はフェンシング部で一番強いでしょう。膝の故障だけがいわゆる弁慶の泣き所だと思います」

(“Sempai wa fenshingu bu de ichiban tsuyoi deshou. Hiza no koshou dake ga iwayuru Benkei no nakidokoro da to omoimasu.”)

[(Addressing a senior student:) “You’re the best in the fencing club, right? Your only Achilles’ Heel, so to speak, is your weak knee.”]