tranquillo
/trahn-KWEEL-loh/ · adjective / interjection
relax / don't worry / calm
look it up on WordReference ↗In context
Tranquillo, ci penso io.
Relax, I'll take care of it.
It means "calm", but on its own it's a reassuring "no worries, it's handled". Make it agree with who you're speaking to: "tranquilla" to a woman.