L. cognZmen, f. co- together + (g)nZmen name; cf. co-gnZ-scere to learn, know.]
1. In Latin use: (a) The third name, family name, or surname of a Roman citizen, as Marcus Tullius Cicero, Caius Julius Cæsar; (b) an additional name or epithet bestowed on individuals, as Africanus, Cunctator (in later Latin called agnomen).
1879 H. Phillips Notes upon Coins 10 Saserna was the cognomen of a noble family which deduced its descent from King Tullus Hostilius.

Hence, in English use:

2. A distinguishing name or epithet given to a person or assumed by himself; a nickname.
1811 L. M. Hawkins C'tess & Gertr. I. 96 Though called by whatever epithets or cognomens imply old age. 1824 Miss Mitford Village Ser. i. (1863) 101 Her father, Jack Bint+was commonly known by the cognomen of London Jack. 1855 Prescott Philip II, I. ii. vi. 213 The cognomen by which Philip is recognized is ‘the Prudent’.

3. An (English) surname.
1809 W. Irving Knickerb. (1861) 157 The name of Alexander+coupled with the gentle cognomen of Partridge. 1867 M. E. Braddon R. Godwin II. iii. 39 The Queen of Beauty was distinguished by the very commonplace cognomen of Watson.

4. loosely. Name, appellation. [So, in Latin, very commonly used by Vergil and other poets, for a name given to a country, river, etc.]
1852 Hawthorne Blithedale Rom. iv, I repeated the name [Priscilla] to myself three or four times+this quaint and prim cognomen+amalgamated itself with my idea of the girl. 1857 Wood Com. Obj. Sea Shore 4 The Common Shag, a bird of a monosyllabic English cognomen. 1872 Jenkinson's Guide Eng. Lakes (1879) 189 A lane, bearing the euphonious cognomen of Spooney Green.