The word Bad meant good when it was first commonly used in 1897, but then the meaning actually meant something that was not okay or "bad". Later the meaning was reincarnated in 2008 to meaning something was good again or good looking. It has stayed the same since then.
The word Bad originated in the 1400's but was rare to be used. It started to get used a tiny bit more in the 1700's. It is most likely to come from the Old English derogatory term baeddel, and it's diminutive baeddling. The meanings of those words probably came from the word baeden which meant to defile.
This word is important to our generating because it's how we express our feelings in a way that our parents and others don't understand what we're saying.