person who first suggested something; person who brought up a matter
Often carries a nuance that the person who first suggested something should take responsibility for it or do it themselves. Commonly used in the phrase 言い出しっぺの法則 (the rule that the one who suggests it has to do it).
See also: 言い出しべ (いいだしべ)
Since you're the one who suggested it, you should try it first.
Since I was the one who raised the matter at the meeting, I'll take responsibility until the end.