plughole
Concise Oxford English Dictionary © 2008 Oxford University Press:
plughole/ˈplʌɡhəʊl/
▶noun Brit. a hole at the lowest point of a bath or sink, down which waste water drains away.
– phrases
go down the plughole informal be unsuccessful, lost, or wasted.
go down the plughole informal be unsuccessful, lost, or wasted.
'plughole' also found in these Oxford entries:

