The most important person, the one who is in charge. / Someone who is top of a ranking, who is ranked first. / Oneself, being considered foremost, as by an egoist.

(idiomatic) To make an extra effort, so as to help or hinder.

(idiomatic) To become serious, organized, worthwhile, etc.

(idiomatic) To profit from a craze; to join a trend.

(idiomatic) To forcibly prevent oneself from speaking, especially in order to avoid saying something inappropriate or likely to cause a dispute.

(idiomatic) To keep someone attentive, active, busy or alert.

(idiomatic) Attentive, active, busy or alert.

(idiomatic) To keep quiet; to keep a secret; to not tell.

Taken onto an airplane (or a ferry, etc.) with a passenger, rather than checked.

That luggage or baggage which is taken onto an airplane (or a ferry, etc.) with a passenger, rather than checked. / A bag, suitcase, etc., used to carry this luggage or baggage.