The Vietnamese word "ngọc hành" refers to the penis. It's a colloquial term that combines "ngọc," which means "jade" (often associated with something precious or valuable), and "hành," which means "to act" or "to perform." This term can be considered somewhat poetic or euphemistic when referring to male anatomy.
"Ngọc hành" is an informal term for the penis in Vietnamese. It's best used in casual conversations and can be seen in literary texts for its poetic quality.