noun which may take the genitive case particle 'no', adverb
fully fledged; competent; proper
Describes someone or something that has reached a respectable, full-fledged level, often with a nuance of 'in one's own right' or 'as much as anyone else'. Used adverbially or with の.
彼もいっぱしの大人になった。
He has become a fully fledged adult, too.
いっぱしの口をきくようになったね。
You've started talking like a proper grown-up, haven't you?
一人前 also means 'full-fledged' or 'adult', but often emphasizes independence or a full portion. いっぱし can carry a slightly ironic or colloquial tone.
The origin is uncertain. It may be related to 一端 (いったん, 'one end'), but the exact derivation is unclear.