The figure 1,001. / A great many; numerous.

(of text) Formatted so that there is one half of a blank line between each line of text

(colloquial, childish) A retort to a negative accusation, implying that the accuser shares the fault.

(UK) A traditional house with one room upstairs and one room downstairs.

plural of one-up-one-down

Each individual should act for the benefit of the group, and the group should act for the benefit of each individual.

Alternative form of it takes one to know one

(idiomatic) A state of dread.

(idiomatic, vulgar, informal) In a state of being unprepared, powerless, or idle.

(idiomatic) astonished, amazed