Sobriquet:
A sobriquet (/ˈsoʊbrɪkeɪ/ SOH-bri-kay) or soubriquet is a nickname, sometimes assumed, but often given by another and being descriptive in nature. Distinct from a pseudonym, it typically is a familiar name used in place of a real name without the need of explanation, often becoming more familiar than the original name. [Wikipedia]
Used in yesterday’s Word Of The Day. How meta.
