1. A word, phrase, or manner of speaking that is no longer commonly used in contemporary language; an expression that has fallen out of fashion or common usage
The phrase 'thee' and 'thou' are obsolete expressions in modern English.
As expressões 'vós' e 'tu' são expressões desusadas no português moderno formal.
2. Language that belongs to a previous historical period and sounds archaic or antiquated to contemporary speakers
His use of 'forsooth' is an obsolete expression that makes him sound like he's from Shakespeare's era.
O seu uso de 'porventura' é uma expressão desusada que o faz parecer de uma era anterior.