Green’s Dictionary of Slang

(US) any large motor vehicle that handles badly.

1941		letter 1 May in P. Jay Sel. Correspondence of Burke and Cowley (1990) 245: Not a single burp or wheeze out of the old Blunder Bus (I am here referring to the Cadillac, not to myself).
1974		J. Scalzo Stand on the Gas! 112: The big blunderbusses aren’t fast enough compared to today’s high modified Chevys [HDAS].
1984		N.Y. Times Mag. 1 Sept. 28: My dad’s got me and one of my friends in the back seat of our old family blunderbuss [HDAS].