[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!"