A Kwyjibo is a move on a Brain Card that would normally be considered cheating. Such actions are extremely common on cards which force a person to come up with words.
This move has its origins in Scrabble where players would frequently make large, non-permitted combinations of letters to play on the board as "words". The name comes from The Simpsons; Bart made the move, and made many points as a result. In 1000 Blank White Cards, any attempt at a false answer to a question or card which requires intelligence, knowledge, or memory is a Kwyjibo, and it is not limited to cards based on Scrabble or other word games.
Kwyjibo words are frequently used when multiple rare consonants, such as Q, J, X, Z, or K, are required to be used in a word for the most beneficial outcome. Also, if rare letters are given point values, a Kwyjibo move will be more common. However, if common letters are given arbitrarily and inexplicably high point values, there is less of a chance of a Kwyjibo. Some variants add all Kwyjibos to the "dictionary", so that a certain Kwyjibo can also be used against you in a later card.