Testicles. Australian rhyming slang deriving from 'Niagara Falls' = balls. Note: I believe the current definition under 'niagras' is an error, and not many people will find this definition unless you spell it exactly like Niagara (falls).
After he broke up with her over SMS, she went to his door and kicked him in the niagaras.
by south mouth 32 June 28, 2013