also: がんね
noun
fast asleep; out cold; dead to the world
Slang term for sleeping very deeply, often to the point of being unresponsive. Similar to 爆睡 (bakusui) but with a slightly more intense or 'knocked out' feel.
See also: 爆睡
昨日は疲れてガン寝しちゃった。
I was so tired yesterday that I crashed out completely.
彼はガン寝していて、全然起きない。
He's out cold and won't wake up at all.
Compound of the slang intensifier ガン (gan, 'extremely') and 寝 (ne, 'sleep'). The exact origin of ガン is uncertain, but it is used in other slang terms like ガン見 (gan-mi, 'stare intently').