Green’s Dictionary of Slang

1. (Aus.) a trick, a hoax.

1945

Baker Aus. Lang.

2. (US) deceptive, untrustworthy.

1898

Ad Sense q. in R. Schorman Selling Style (2003) : Ad journals urged merchants to avoid ‘pyrotechnic English’ and shun ‘“flumdoodle” advertising’ and to rely instead on ‘honest, earnest, heart-to-heart statements and [...] strong though dignified typographic display’.

3. an otherwise unnamed thing.

1999

Epinions.com Nov. 🌐 If some bozo made a million bucks off of selling you (and many other people) a poorly-designed flumdoodle, couldn’t you make a million and one bucks selling an intelligently designed flumdoodle.