(Choumon no isshin; “A needle in the scalp”)
A warning, scolding, or prohibition that really hits home. An admonition that feels like a needle stuck into the “vital point” at the top of the head. Being told off so accurately and sharply that it hurts.
This one’s a noun phrase, and the primary noun is combined with a number as 一針 (isshin) “a single needle.” Since Japan doesn’t require singular or plural to be specified, the number here acts to emphasize the kind of pain we’re invited to imagine: a single piercing point. This noun is joined by the associative particle の (no) to another noun: 頂門 (choumon) – literally meaning “peak gate,” but on the concrete level, referring to the top of the head.
The reference here is to acupuncture – a practice in Chinese traditional medicine in which needles are inserted into designated points in the body in the belief that doing so would influence the flow of vital energies. This kotowaza seems to have a complementary double-meaning, then: first, the image of a “sharp” admonition works almost regardless of culture or language. But second, if you believe in the efficacy of acupuncture, then it probably seems natural for a specific “gate” being pierced to have a therapeutic effect.
一針 may sometimes be written as 一鍼 and/or pronounced as hitohari. However, replacing 頂門 with 頭頂 (touchou), a more literal way to say “the top of the head,” is an error.
(Itsumo doori ni kaasan ga shukkin shita. Demo, kyou wa choumon no isshin no you na hitokoto wo nokoshite dekaketa.)
[Mother went to work as usual. However, today on her way out, she left behind a single, stinging word of warning.]