Placed in a situation where one's best behaviour is necessary or advisable.

As far as concerns one.

(UK, military, slang) A first lieutenant in the navy.

With the consent of all; unanimously.

(automotive) The first finished, saleable car of a new model to come off the production line.

(UK, historical) Cheap seats in a theatre, costing one shilling and nine pence.

(now regional, dated, idiomatic, slang) Not a single one, none (of a number of people or things). / (now regional, dated, idiomatic, slang) Not; with a singular pronoun, negates the clause.

Having come to the end of one's resources. / Used other than figuratively or idiomatically: see on, one's, back.

Having one foot.

(idiomatic, transitive) To compel someone or something to conform to one's desires.