Green’s Dictionary of Slang

(US) something notably large of its type, often as old baster.

1899–1910		G. Wasson notebooks in AS XLIX:1/2 (1974) 61: It was a baster of a rat.
1940		Harper’s Mag. 107/1: Baster [...] is a popular word with boys [...] He’s an old baster, they say, when they pull an eel out of an eel trap.
1975		J. Gould Maine Lingo 3: Baister—Possibly Baster [...] indicating large size [...] Often with the adjective ‘old’.