Emily Dickinson Lexicon

SIR, n. [sur; Fr. sire, and sieur, in monsieur; Norm. sire, lord; Corn. sira, father; Heb. שור, shur, to sing, to look, observe, watch, also to rule. The primary sense is to stretch, strain, hold, &c., whence the sense of a ruler or chief.]

1. A word of respect used in addresses to men, as madam is in addresses to women. It signifies properly lord, corresponding to dominus in Latin, don in Spanish, and herr in German. It is used in the singular or plural. Speak on, sir. – Shak. But, sirs, be sudden in the execution. – Shak.
2. The title of a knight or baronet; as, Sir Horace Vere. – Bacon.
3. It is used by Shakspeare for man. In the election of a sir so rare. [Not in use.] – Shak.
4. In American colleges, the title of a master of arts.
5. It is prefixed to loin, in sirloin; as, a sirloin of beef. This practice is said to have originated in the knighting of a loin of beef by one of the English kings in a fit of good humor. – Addison.
6. Formerly the title of a priest. – Spenser.