Bet - "An expression that means 'I agree', 'good news'."

Bet  this essentially means yes,  I'm down,  or some iteration of agreement to a task. Use it in a sentence: I'll be there. Bet.  It's often just a one word reply: are you coming tonight?  Bet!  Some say it can be used to reflect disbelief as well but I don't have the energy to look into that.