From a Washington Irving character, it's a nickname for a New Yorker, Dutch-descended or otherwise
In its seventeenth issue, dated November 11, 1807, Irving affixed the 
nickname ... his Dutch historianpublished A History of New York on December 
6, 1809,... [DOC] [TLE] The Legend of Sleepy Hollow - Wikipedia [PAR] "The Legend of Sleepy Hollow" is a short story of speculative fiction by American 
author Washington Irving, contained in his collection of 34 essays and short 
stories entitled The Sketch Book of Geoffrey Crayon, Gent.