From the Old Japanese root *naru* (成る), meaning "to become." The potential/intransitive suffix *-reru* shifts the meaning from active becoming to the state of having adapted. Related to 慣らす (*narasu*, "to accustom someone to something"), its transitive counterpart.

慣れる is deeply embedded in Japanese social life. The phrase 慣れ合い (*naregai*) — becoming too cozy or complacent with someone — carries a slightly negative nuance, suggesting a loss of healthy tension. In workplace culture, new employees (新入社員 *shin'nyū shain*) are expected to 慣れる to the office environment quickly, and seniors often use the phrase まだ慣れていない (*mada narete inai*, "still not used to it") with understanding rather than criticism.