From LOTR-TCG Wiki
Whiff is an informal game term. A whiff is when you play a card with a random effect and get no value from it, usually because the card affects cards in hidden area, like a player's deck or your opponent's hand. If you play Forearmed (6U16) and reveal a card with a zero twilight cost from the top of your deck, it whiffed.
The term comes from sports, where a "whiff" is the sound made when you swing a bat or a golf club at the ball and don't even make contact.