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.
彼は早起きに慣れっこだ。
He's used to getting up early.
もう失敗には慣れっこになっちゃった。
I've already gotten used to failing.
慣れる is the standard verb meaning 'to get used to'. 慣れっこ is a more colloquial noun/adjectival noun emphasizing a state of being thoroughly accustomed.
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.