1to suddenly decide to do something; to get the idea of doing something
昨日、急に旅行に行こうと思い立った。
Yesterday I suddenly decided to go on a trip.
彼女は新しい仕事を探そうと思い立った。
She made up her mind to look for a new job.
思い立ったが吉日だ。
Literally: The day you decide is a lucky day. (Strike while the iron is hot.)
会議中に彼は新しい計画を思い立った。
He came up with a new plan during the meeting.