‘惯于’ is a verb phrase meaning ‘to be accustomed to (doing something)’ or ‘to be used to (doing something).’ It combines the character 惯 (guàn), which means ‘habitual’ or ‘accustomed,’ and 于 (yú), a classical preposition that introduces the activity or state one is accustomed to — here, it functions similarly to ‘in’ or ‘at’ in English constructions like ‘accustomed to doing.’ Though 于 appears as a standalone character, in this fixed expression it does not carry its usual locative or comparative meaning but serves a grammatical role marking the object of habituation.

This phrase is moderately formal and often appears in written Chinese or elevated spoken contexts — such as essays, news reports, or thoughtful personal reflection. It typically precedes a verb (often in its bare form, without aspect markers like 了 or 过) and implies long-standing, ingrained behavior rather than recent or temporary adaptation. It carries a neutral-to-slightly literary tone and is rarely used in casual conversation, where simpler alternatives like 习惯（了）are preferred.