noun, noun which may take the genitive case particle 'no', na-adjective
being used to; getting used to; being accustomed to
Colloquial noun/adjectival noun expressing that one is thoroughly accustomed to something, often with a nuance of being so used to it that it no longer feels special or bothersome. Commonly used in the pattern 〜に慣れっこだ or 〜に慣れっこになる.
He's used to getting up early.
I've already gotten used to failing.
Rarely used kanji form; 慣れっこ is strongly preferred.
Derived from the verb 慣れる (to get used to) with the colloquial suffix っこ, which adds a familiar or emphatic nuance. The exact historical derivation is uncertain, but the form is well-established in modern casual speech.