It stands for put reverse knock-in; a reverse knock-in option (RKI option) that becomes in the money (ITM) once the defined barrier (for a put option, it is the barrier set below the strike) is hit and breached (where the option activates). It is a reverse down-and-in put that activates (comes into life) when the barrier is in the money with respect to the strike price.

If the barrier is not broken through (from above), the knock-in event does not take place and the option turns into a standard option (standard put).