You're in the army now,
You're not behind a plow;
You'll never get rich,
Diggin a ditch
You son-of-a-bitch,
You're in the army now

And "foss" = "fossa" = ditch, trench!

fosse or foss
n.
5ME < OFr < L fossa < fossa (terra), dug (earth) < fossus: see FOSSIL6 a ditch or moat, esp. one used in fortifications