Wash

1. To spread the cards face down on the table and mix them up before shuffling the deck

Origin

Mixing up the cards in this way "cleans" the deck of possible non-randomness.

Reason

Washing the deck helps prevent cheating by making it extremely difficult for the dealer to set the deck in such a way so as to allow him to deal predetermined hands. It also helps to simply randomize the deck more, just in case the dealer does not shuffle well.

When is it Done?

Decks are typically only washed when a new deck is put in play. Since new decks are usually arranged sequentially, washing is a good way to quickly rearrange the cards before shuffling to ensure that a) no cards are stuck together and b) cards are harder to track through the deck if the dealer has an easy-to-follow shuffle. Players are allowed to ask the dealer to wash the deck at any time, but this almost never happens.