This man is pounding the husks off a species of nut (Pangium edule). Each grapefruit-sized husk contains several nuts in separate shells. Because the nuts are poisonous in their natural state, people boil them in their shells, then split them open, slice up the nut meats, and leach the slivers of nut meat in a stream before eating them. The nut meat is oily and highly nutritious.
Endicott Image Collection
Nuts -- Malaysia -- Kelantan, Batek (Malaysian people) -- Food., Lebir River (Malay)