Slang for "Phenazepam", a benzodiazepine drug similar to Xanax (Alprazolam) and Valium (Diazepam). A typical dose is 1mg which is a bit stronger than a 10mg Valium or 1mg Xanax Bar. They are usually sold as 0.5mg or 1mg capsules/pills.
Phennies are quite strong so be wary. It is recommended not to go over 10mgs/day.
Friend A:"Got any bars?"
Friend B:"I've got phennies and they're bangin'! Here's one."
*30 minutes after popping*
Friend A:"Daaaamn, this is chillin'!"