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Australian slang.
Two bob was once known as two cents in currency, and a watch is an instrument to measure time .
In todays terms ,together they form " a 2 cent watch" .
Would you buy a watch for 2 cents ? and how good would you expect it to perform?. probably very erratically.
Hence the ocker term "as mad as a two bob watch"

related - also see Shithouse Rat and cut snake.
Laurie says " keep an eye on Wade "
scott inquired " why"?
dan says " he's as mad as a two bob watch" !!
by wackyv8 June 17, 2008