‘Bryn’ has supplied a verifiable example in this sense from 1985.
In colloquial use, chiefly in North America, the word backwash can refer to the unappetizing mixture of saliva and beverage that flows back into a glass or bottle after a person takes a drink. This usage seems to be relatively recent; the earliest evidence for the sense found by OED researchers thus far dates to 1988:
It is late afternoon and the wet April air covers suburban pavement like the slimy backwash coating the glass of the shared Coke bottle your best friend meretriciously let you drink from in sixth grade.
1988 Washington Post 11 Sept. x., p. 15
However, anecdotal evidence suggests that this use of backwash may in fact date from at least the early 1970s. If this is true, it is likely that backwash (like many colloquial and slang words) first took hold primarily in oral use, and only later gained enough currency to be found in published texts. Given the amount of time between the early 1970s and the Washington Post article, it seems likely that documentation of backwash in this sense may be found prior to 1988. Can you help us find earlier evidence?
Update: The OED records this word in use from the late 19th century in the more general sense ‘the motion of a receding wave, a backwards current’. The sense we are interested in for this appeal is specifically the slang usage.
Posted by OED_Editor on 18 September 2013 14.56