[Australian slang]
Also, generally exclamatory.
Two distinct uses.
1. Profound or vehement agreement. eg.
A: "That was unexpected!"
B: "Fuckin' Oath!"
2. An almost old fashioned swearing to the truth of something; equivalent to, "by my Oath," or, even, "upon my Honour." eg.
A: "You were there? You HEARD him say it?"
B: "Fuckin' Oath!"