Question: what cool words start with j?
Answer: JABBLE. An old Scots word meaning “to shake liquid around in a container.”
JACK-AT-A-PINCH. Seventeenth century slang for someone (originally, a clergyman) called in as a replacement at the very last moment.
JACK-SHARP. ... 
JACTANCE. ... 
JACTITATE. ... 
JADOO. ... 
JALOUSE. ... 
JAMBLE.