Noun, Noun which may take the genitive case particle 'no', Na-adjective (keiyodoshi)
being used to ...; getting used to ...; being accustomed to; being conditioned to; be familiar with
私達はみんな夜遅くまで働くのに慣れっこになっている。We are all accustomed to working late at night.
Also written 馴れっこ【なれっこ】