[number one] n (ca. 1705) 1: one's own interests or welfare: oneself "looking out for number one"--often written No. 1
2: one that is first in rank, importance, or influence--often written No. 1
[2]number one adj (1839) 1: first in rank, importance, or influence: foremost "cancer is the country's number one killer"--often written No. 1
2: of highest or of high quality