- noting a vessel regarded by a shipping-classification society as being equipped to the highest standard and with equipment maintained in first-class condition.
- Also A number one. Informal. first-class; excellent; superior: The meals there are A one.
in figurative sense of “first-rate,” 1837, in Dickens; from Lloyd’s of London designation for ships in first-class condition (with the letter referring to the condition of the ship and the number to that of the stores).