tranquilo
/tɾanˈkilo/ · A1 · adj
calm, peaceful, tranquil, quiet
🍵 tranquilo = tranquil + o: same Latin root, tranquillus (calm)
English tranquil and Spanish tranquilo both come from Latin tranquillus. Swap the ending for -o (or -a for feminine) and you have it. Said alone, ¡Tranquilo! means relax, calm down.