tergum

Tergum, tergi, n. Second declension neuter. Two phrases carry most of its weight. Terga vertere, to turn the backs, is the standard Latin for a rout, and a tergo on its own means "from behind, in the rear", which is where a Roman general least wants to be attacked.

The second sense, hide or leather, is the same word seen from the animal's side: a shield of seven ox-hide layers is Virgil's septemplex tergum. Note that tergum is a body's back, not an object's: the rear of a building is pars postica or aversa. And the verb tergeo, tergere, "to wipe clean", which gives detergent, is a different word that only looks related.