Accustomed or habituated to. This expression is often put as be or get used to, as in I'm not used to driving a manual-shift car, or She can't get used to calling him Dad. [Early 1500s]
Formerly. This sense is used with a following verb to indicate a past state, as in I used to ride my bicycle to the post office, or This used to be the best restaurant in town. [Late 1800s]
I would call it an idiom in modern English, but it does relate to an archaic use of the verb "use", meaning "to observe or follow as a custom". The "to" was originally the particle that preceded a following infinitive.