Originally from martial arts, 'tap out' means admitting defeat by tapping the opponent's body. Watching the championship, I saw the fighter tap out to avoid injury.
Pro Tip 2/3
Metaphoric Use
Figuratively, 'tap out' can indicate a person is completely overwhelmed or giving up. I had to tap out after studying for 12 hours straight.
Pro Tip 3/3
Informal Setting
'Tap out' is casual and usually not used in formal contexts. After the long hike, she jokingly said she was ready to tap out.