Green’s Dictionary of Slang

1. (orig. US) pertaining to the tables nearest to the stage in a nightclub or similar establishment, also attrib.; thus ringsider, one who sits at such a table.

1926		O.O. McIntyre New York Day by Day 9 Mar. [synd. col.] Ziegfeld has a show here [...] with ringside tables $11 a seat.
1940		J.H. O’Hara Pal Joey 82: They had this table ringside.
1944		E. Wilson 30 Mar. [synd. col.] Sophie Tucker [...] singing for Helen Keller at the ringside at the Copacabana [...] when Miss Keller was introduced some of the ringsiders’ eyes filled with tears.
1975		E. Torres Carlito’s Way 59: Right away they was ringside at all the Latin joints.
2001		(con. 1964–8) J. Ellroy Cold Six Thousand 337: He caught Barb’s tenner. He stood ringside. He killed time.

2. in a restaurant, adjacent to one’s table.

2021

T. Swerdlow Straight Dope [ebook] [T]he buffalo-headed waiter was ringside, holding his pad in a brown hand thick as the filet,.