OED gives 'stolid' as:
'Abounding in beef; resembling beef; fleshy; obese; stolid or brawny.'
together with this example from Samuel Smiles' 'Self-Help' (1859):
"This dunce had a dull energy and a sort of beefy tenacity of purpose."
I suspect there's a connection with terms such as beef-headed and beef-witted.