expression, i-adjective
cannot tell one thing from another; cannot distinguish (e.g. good from bad)
Used when someone loses the ability to make distinctions, often due to strong emotion, intoxication, or obsession. Commonly appears in negative form 見境がつかない.
See also: 付く (つく)
彼は怒りで見境がつかなくなった。
He lost the ability to distinguish right from wrong in his anger.
酔っ払って見境がつかない状態だった。
He was so drunk he couldn't tell one thing from another.
見境 is a noun meaning 'discernment' or 'distinction', often used in negative contexts like 見境なく (indiscriminately). 見境がつかない is a fixed expression built on it.
Compound of 見境 (discernment) + が + つかない (negative potential form of つく, here meaning 'to be established/perceived'). The exact historical derivation is uncertain, but the phrase is a conventionalized expression.