An allusion to the practice of some large primates of walking on all fours with their knuckles close to the ground.

knuckle dragger (plural knuckle draggers)

(idiomatic, often derogatory) A large, strong, and rather dimwitted person.
- 2008 December 21, Pat Hickey, “Prepare the Pulitzer”, in Chicago Daily Observer, retrieved 5 Jan. 2009:
  Mickey was wide receiver on the St. Rita Football Team in 2007—a big strapping, robust south side knuckle dragger and chick magnet.
- 2017, A.K. Brown, Humans on the Menu (Champagne Universe Series: Book 2), page 35:
  The scientists were frustrated by the constant need to have knuckle draggers following them like mother hens.

“knuckle dragger”, in OneLook Dictionary Search.
“knuckle dragger, knuckle-dragger, knuckledragger”, in Google Books Ngram Viewer.

English