හරි ගිය එකාට කෙලවන්න උත්සාහ කරන එකාට තමයි backstabber කියන්නෙ.
Your definition is wrong machan.
he act of saying harmful or unpleasant things about a person when they are not present is backstabbing.
sources : http://dictionary.cambridge.org/dictionary/english/backstabbing