1. [adj]  characterized by toilsome effort to the point of exhaustion; especially physical effort; "worked their arduous way up the mining valley"; "a grueling campaign"; "hard labor"; "heavy work"; "heavy going"; "spent many laborious hours on the project"; "set a punishing pace"
2. [adj]  resulting in punishment; "the king imposed a punishing tax"

PUNISHING is a 9 letter word that starts with P.