Comes from when the Japanese bombed Pearl Habor in sneak attack fashion.
From the Malay phrase "penampar Jepun" to describe harsh treatment by Japanese soldiers during the Malaysian occupation of WWII.
2. (noun)A painful slap across one's face.
Don't make me japslap you!
Did you see how hard that dude japslapped him?
I'm about to japslap that bitch!