Green’s Dictionary of Slang

duff-trap n.

[SE duff, a form of pudding + trap n.1 (5)]

(Aus.) the mouth.

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[Aus]Bulletin (Sydney) 20 Aug. 14/2: Bill [...] tackled the sea-fowl in a desperate spirit of bravado. In about five minutes he made a bolt, with a huge, tarry hand obscuring his ‘duff-trap,’ making a queer, guttural sound in his throat as he scooted. He communed with Mr. D. Jones for some time.