The filmy, cloudy residue at the bottom of a drink that you get when you pass a drink to someone else. Always seems to be at the bottom, separate from the rest of the drink, like oil and water.
Also a good way to determine if someone has eaten fish or tomatoes recently.
Soph: Can I have my coke back, now?
Tat: Yeah, here you go.
Soph: Is that backwash at the bottom?
Tat: Er...
Soph: You keep it.

(incidentally, if anyone gets a bottle with backwash in the bottom, try refilling it with its original liquid as well, so it's full, then heating the bottle from below. Because it's seperate, I wonder if it behaves like a lava lamp. If anyone wants to try this, feel free to enlighten me.)
by ANBY June 17, 2004
Backwash is often created inadvertently or unintentionally when liquid escapes the mouth during the process of drinking, though it can also be created intentionally by an act of spitting or purposely letting liquid fall from one's mouth and back into the container.

When multiple people drink from the same container, there will usually be some amount of backwash put back into the container; as the number of sips increases, in general, the concentration of backwash also increases.
"Dude, I don't want your backwash." (This person has perhaps declined someone's offer of their nearly empty can of Coke).
by Hari____ April 11, 2006
to take a mouthful of drink from a container (cup/glass/bottle) and spit it out back into the container in which saliva and bits of food go back in with the drink
Someone eats a carrot sandwhich and becomes an arsehole by taking a mouthful from their cup and spitting it back in which bits of bread and carrot go into the drink with saliva=backwash
by bread infection October 21, 2005
Backwash is when particals in your mouth, (eg. food) are pulled back into the bottle by reverse suction.
Oh man, check out the backwash crackers in that bottle.
by Tim Wells September 13, 2005
drinking something and spitting it back out into the bottle
eww you backwashed that, i'm not gonna drink that!
by j00l.i.e April 26, 2003
The transfer of saliva and/or foreign substances from one's mouth through a liquid medium.
Melissa's soda has back wash in it, so i do not want it again!
by Prodigi January 18, 2009
The sediment and/or particles of whatever you're eating that ends up in your drink.
William- "Hey Sven, what are those floatin' things in your cup of Perrier?"

Sven- "Just some backwash of water chestnut and sesame"

William- "Why on gods green earth were you eating water chestnuts and sesame seeds?"

Sven- "I.. dont.. know..."
by The_Buddy August 17, 2011

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