When you tell someone to do something either really important. Also when you tell someone a really big secret and don't want anyone to know.

When you say you got down you are asking someone do they have next in a pick up game

It's what your freinds tell you when you aren't ready to go or do something.

when someone tells you this, they are trying to say that you can't do something cuz you're not brave. so you have to reply "i'm down", and then do it

An Arnold Schwarzenegger quote showing Arnie himself armed with a gun shouting those very words. Typically meaning "Lie down or I kill you", it could scare someone into lying down because they value their life.

A contemporary version of the old expression, "don't let the bastards get you down," this version stands for "don't let the motherfuckers get you down." The mofos are generally anyone in authority or authority figures, whether bosses, family members, partners etc.

Phrase refers to a song by Tool called Aenima. Used appropriately it basically means:

"I'll see you in Hell." or "I'll see you at the End of the World."

Can be used as a compliment, or an insult, depending on its use. Maybe more prolonged use will define it more.