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.

Alternative form of no questions asked

(often hyphenated) Characterized by a pledge to make no inquiries concerning the motives, circumstances, or identity of the person receiving an agreed benefit or performing a proposed action.

(of an agreement, promise, etc.) Without inquiring into the motives, circumstances, or identity of the person receiving a benefit or performing a proposed action.

Alternative form of no strings attached

(espionage) The assumption of a covert role in an organization which does not have ties to the government of the operative in question.

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

Alternative form of no-fly list

(law, aviation) A confidential list containing the names of people who are not permitted to board an aircraft, prepared by government authorities in various jurisdictions and distributed to commercial airlines.

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