(3 syl.). The usages of polite society. The word means a ticket or card, and refers to the ancient custom of delivering a card of directions and regulations to be observed by all those who attended court. The original use was a soldier's billet. (French, etiquette; Spanish, etiqueta, a book of court ceremonies.)
“Etiquette ... had its original application to those ceremonial and formal observances practised at Court ... The term came afterwards ... to signify certain formal methods used in the transactions between Sovereign States.” —Burke: Works, vol. viii. p. 329.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894