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< bum, rear end (Anglo) + twaddle, verbal drool (see tweedle-dum and tweedle-dee).
A serendipitous word, allusive and proto-onomatopoeic (sounds like twäæt, hence its utility as flambic, q.v. flambics and flambic emoticons) despite its rather impossible model of human anatomy.
The word is used in advanced flame-blogging, albeit rarely due to its obscurity.
Bumtwaddle obtained 40 Google Search hits on 2012.12.28. The first verified, validated usage was in 2003 by [merzbunny] at an advanced (i.e., literate and intelligent, though maintaining a debased and degradatory standard) website  as follows:
“It was a toss-up between (using the phrase) "the summit of wank" and "the apogee of bumtwaddle," but reason won out in the end.”
Compare Russ. моя задача кульваны и серый, approx. my occupation is sacklike and grey
Unrelated to bumfuzzle, a verb meaning to perplex.