atrás
/aˈtɾas/ · A1 · adv
back in time, ago
🔙 a + tras: to + behind, from Latin trans (across, beyond)
Tras means behind or after, and the a- adds direction: toward the back. That covers all three uses: place (quedarse atrás, stay behind), time (años atrás, years ago) and the command ¡Atrás! (Get back!).