He was always calling him “number one” even though the captain is the actual #1 officer in a ship. So….
Literally: only – one – not – two
Alternately: Unique. “The one and only.” There’s nothing else like it in the world; often with the implication that the thing being described is irreplaceable and precious.
Notes: This self-explanatory phrase is technically a compound of synonymous terms, added together for emphasis. It has relatively many yojijukugo synonyms, including the I’ll-do-you-one-better 無二無三 (muni musan or muni muzan), “not two and not three,” and 唯一不二 (yuiitsu fuji; be careful!).
One thing to watch out for: while 空前絶後 and 史上空前 also refer to uniqueness, they focus on the singular nature of something compared to the historical sweep of time, while today’s phrase focuses on something’s value due to its singular nature “in the world.”