ichidan verb, intransitive verb
be presumptuous; be forward; act clever beyond one's years
Describes someone, often a child or young person, who acts overly clever, forward, or presumptuous in a way that can be annoying or inappropriate. The verb is intransitive and often used in negative or critical contexts.
あの子はさいはじけていて、大人の会話に口を出す。
That child is presumptuous and butts into adult conversations.
さいはじけるのも
Stop being so forward and presumptuous.