Mr

Mister, usually written in its abbreviated form Mr. (US & HK) or Mr (US & UK & HK), is a commonly used English honorific for men under the rank of knighthood. The title derived from earlier forms of master, as the equivalent female titles Mrs, Miss, and Ms all derived from earlier forms of mistress. Master is sometimes still used as an honorific for boys and young men, but its use is increasingly uncommon.

The modern plural form is Misters, although its usual formal abbreviation Messrs(.) derives from use of the French title messieurs in the 18th century.Messieurs is the plural of monsieur (originally mon sieur, "my lord"), formed by declining both of its constituent parts separately.

Historically, mister—like Sir or my lord—was applied only to those above one's own status in the peerage. This understanding is now obsolete, as it was gradually expanded as a mark of respect to those of equal status and then to all gentlemen. It is now used indiscriminately.

In past centuries, Mr was used with a first name to distinguish among family members who might otherwise be confused in conversation: Mr Doe would be the eldest present; younger brothers or cousins were then referred to as Mr Richard Doe and Mr William Doe and so on. Such usage survived longer in family-owned business or when domestic workers were referring to adult male family members with the same surname: "Mr Robert and Mr Richard will be out this evening, but Mr Edward is dining in." Such usage today is rare in American culture but still quite common in others as a sign of respect when first names are being used, the last name is not known, or where English is not the mother tongue.

...
Wikipedia