The latter sense is depreciated for anything outside of entomology, in part because a testudo is a tortoise. As for the etymology, it was borrowed in 1646 from Latin testāceus, shell-covered, from testa, a shell or earthenware pot, and by extension a terra cotta brick + an adjectival ending. I confess, it's not as much fun to say as testudo, especially once you know about the testudo formation, but it's still a worthwhile state in an embattering world.
When The Descent of Man Charles Darwin described the male of a species of ant as testaceous, he meant it both ways.
---L.
