剛剛在 free-definition 找到它的定義...

(手榴彈附註解釋)
http://www.free-definition.com/Grenade.html

When a grenade is dropped in to a enclosed space like a tunnel, the dropper is usually supposed to yell, "Fire in the hole" to alert his comrades that an explosive is about to detonate.

(當丟手榴彈的人把手榴彈丟進一個封閉空間或是隧道之類的地方, 丟手榴彈的人通常都會大喊一聲 "Fire in the hole" 來警告夥伴們有東西要爆炸啦!)

PS: free-definition 是一個非常好用的英文定義查詢網站, 大家可以多多利用!