He was captured, put in a Vietnamese prison camp.
6 seconds sound clip from the Pulp Fiction movie soundboard.
You can hear this line at 01:07:15 in the Blu-ray version of the movie.
Quote context
[...]
- A man he'd never met before in his life...
- To deliver to his infant son who he'd never seen in the flesh, his gold watch.
- Three days later, your granddad was dead, but Winocki kept his word.
- This watch was on your daddy's wrist when he was shot down over Hanoi.
- He was captured, put in a Vietnamese prison camp.
- He knew if the gooks ever saw the watch it'd be confiscated, taken away.
- The way your dad looked at it, this watch was your birthright.
- Five long years he wore this watch up his ass.
[...]