Green’s Dictionary of Slang

1. something large and unruly.

1867		G.E. Clark Seven Years of a Sailor’s Life 149: Here, Ned, bend this on; he is a lunker, and will give us a hard try.
1880		Wellington Enterprise (OH) 1 July 4/1: Isn’t it a lunker, big enough for a grave-yard fence!
1987		C. Hiaasen Double Whammy (1990) 21: One of those freakishly big bass known as lunkers or hawgs.

2. a dilapidated motor car.

1973

Seattle Times 2 Dec. E5: The development of small automobiles that make far more efficient use of gasoline than the lunkers we drive now [HDAS].