One of the mysteries of the English language finally explained.
Trousers which reach to, or almost to, the ankle.
boy (or child) in long trousers
: an immature person, or one who is behaving in a childish or childlike manner.
in long trousers
: (as postmodifying phrase) in a more mature, developed, sophisticated, etc., form.
Mid 18th century. From long + trousers.
long trousers/ˌlɒŋ ˈtraʊzəz/
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