1The person who first brings something up; the one who suggests or raises an idea (colloquial).
誰がこの旅行を提案したの？言い出しっぺは田中さんだよ。
Who suggested this trip? Tanaka was the one who brought it up.
言い出しっぺが幹事をやることになった。
The person who suggested it ended up being the organizer.
それを言い出したのは君だから、言い出しっぺとして責任を持ってね。
You were the one who suggested it, so take responsibility as the originator.