Number One 1 n. phr. , informal Yourself; your own interests; your private or selfish advantage. Usually used in the phrase "look out for number one". He was well known for his habit of always looking out for number one.
number one 2 adj. phr. 1. Of first rank or importance; foremost; principal. He is easily America's number one golfer. 2. Of first grade; of top quality; best. That is number one western steer beef.