Children should behave well and be quiet, especially in the presence of adults.

Used to indicate that somebody does not wish to be disturbed, e.g. a sign on a hotel room door, or a "busy" mode of an instant messenger.

(espionage) An announcement of dismissal of an agent or source an intelligence agency considers to have become unreliable.

Indicates that an area should not be entered, e.g. a military installation, or as a sign on a hotel room door.

Barely useful; almost worthless or pointless, but nevertheless better than having nothing at all.

(UK) A note from a doctor certifying the patient is ill, and therefore unable to go to work, school etc. / (UK, idiomatic) Someone who dodges work because of (feigned) illness.

A system used by dentists for associating information with a specific tooth.

Alternative form of the apple does not fall far from the tree

Alternative form of the apple does not fall far from the tree

(informal) Very little space (available) (of a very small room).