To light a cigarette, pipe, etc.
To loosen, slacken, or ease off.
To shock (someone) with a stun gun.
He saw Mary and his face lit up.
To open fire on a target or group of targets, especially with rockets, a flamethrower etc.
To illuminate, to bring light to something, to brighten.
To show an increase in activity or a brightening of mood.
Smoking in this building is not allowed, so I always step outside to light up.
To make happy.
To activate the emergency sirens on a police vehicle in order to pull someone over.
To create a flame (using a match, flint, etc.).