(Benkei no nakidokoro; “The place that makes Benkei weep”)
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.
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.”
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.
(“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.”]