When you are trying to think of a word but your mind locks on to a similar word preventing you from remembering the desired word.
Chuck - "We had some of those New Orleans pastry things - oh, darn, I had it on the tip of my tongue but now all I can think of is 'bidet' and I know that's not it."
Carol - "Bidet!? It's a beignet - You don't eat a bidet you goof!"
Chuck - "I know, I know - I said that's not it - I had word lock."