The Vietnamese word "ông ba mươi" colloquially means "tiger." It is often used in informal conversations and is derived from the literal translation of "ông" (Mr.) and "ba mươi" (thirty), but in this context, it refers to the animal itself.
In summary, "ông ba mươi" is an informal term for "tiger," used in everyday conversation to describe the animal or metaphorically refer to someone with strong or fierce traits.