Green’s Dictionary of Slang

(US) a positive intensifier, very, extremely, usu. referring to food.

1905		J.W. Carr ‘Words from Northwest Arkansas’ in DN III:i 84: Jumpovers and ’lasses are larrapin’ good.
1906		A. Adams ‘The Double Trail’ Cattle Brands 🌐 Young calves, you know, make larruping fine eating for grays [i.e. wolves].
1940		‘Digg Mee’ ‘Observation Post’ in N.Y. Age 25 May 10/4: His larruping, dynamic Np. 14 has met with his Waterloo.