Green’s Dictionary of Slang

1. (orig. US milit.) to shirk, to evade one’s duties.

1946		‘Sl. of Maladjustment’ in AS XXI:3 Oct. 238/2: flubbin’ the dub. Loafing, goldbricking.
1947		D.W. Hamilton ‘Pacific War Lang.’ in AS XXII:1 Feb. 55: To flub off, to flub the dub. To escape assigned duty.
1966		12 TAC FTR WG Song Book 25: When a bomber jockey walks into our club / He don't drink his share of suds / All he does is flub his dub.

2. (US) to masturbate.

1946		‘Sl. of Maladjustment’ in AS XXI:3 Oct. 238/2: flubbin’ the dub. [...] obscenity.
1966		F. Elli Riot (1967) 190: Hell, man, I’m desperate. I’ll be flubbin’ my dub in here for the next twenty years.
1975		L. Dills CB Slanguage.

3. (US) to blunder, to fail in a task.

1959		L.P. Boone ‘Gator Sl.’ AS XXXIV:2 156: He may fail; in his words, he flubbed, flubbed up, flubbed the dub, or made a boo-boo.
1974		J. Lahr Hot to Trot 197: But John flubbed the dub. He cared for this girl.