已久 is an adverbial phrase meaning 'for a long time already' or 'for quite some time'. It combines 已 (yǐ), meaning 'already', and 久 (jiǔ), meaning 'long (time)'; together, they emphasize that a state or action has been ongoing for an extended period and continues up to the present. Though grammatically concise, 已久 is relatively formal and literary — it appears more often in written Chinese (e.g., essays, news reports, or classical-influenced speech) than in casual conversation.

This phrase typically modifies verbs or adjectives and often follows the subject directly, sometimes preceded by a time marker like 自从 (since) or after a clause indicating duration. It conveys a sense of endurance, persistence, or even mild resignation — for example, describing a prolonged wait, an unchanged situation, or a longstanding condition. Unlike colloquial alternatives like 很久了, 已久 carries subtle weight and gravity, suggesting the passage of time has significance to the context.