What it means
ってば is an emphatic sentence-ending expression used to insist that the listener accept, remember, or act on what the speaker has just said. Depending on the preceding clause, it can mean “I said ...,” “I told you ...,” or “I’m telling you ... .” It conveys impatience or strong insistence and is especially common when repeating a statement, denial, warning, or request.
Usage note
ってば is strongly conversational and does not have a natural polite variant of the same grammatical construction. For formal or respectful speech, use a more explicit expression such as と言いましたよ or とお願いしましたよ instead.
Example sentences
締切は来週の金曜日だってば。
I told you, the deadline is next Friday.
その件はもう上司に報告したってば。
I told you, I already reported that matter to my supervisor.
窓を閉めてってば。
I told you to close the window.
鍵は机の引き出しに入れたってば。
I told you, I put the key in the desk drawer.