Definition of "Sebastian"

Of Messaline: Sebastian was my father; / Such a Sebastian was my brother too, / So went he suited to his watery tomb.

c. 1601–1602 (date written), William Shakespeare, “Twelfe Night, or What You Will”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act V, scene i]

I was hoping the man would have some romantic name like Sebastian or Julian. However, as a surname Fletcher's all right.

1968, Robin Maugham, The Second Window, McGraw - Hill, page 275

“She was a new student. I was a counterculture hero. Back then I went by a different name. Sebastian. Sexy, right? And so much better than Guy. You can't be a counterculture hero and a Guy. That name is way too average. […]

2016, Nathan Hill, The Nix: A novel, Vintage, page 596