Definition from Wiktionary, the free dictionary
clue-by-four (plural clue-by-fours)
- (informal, figuratively) An object used when roughly imparting clue to the clueless.
- 1993, Anonymous, Re: Recent Events from the Horseman, alt.sex.bondage, 
- If I think they're a jerk who's using a march to show how cool and open minded they are, I pull out the ol' clue by four.
- 1998, TCurryFan, Re: Carlos and Ashley?, alt.fan.power-rangers, 
- Nah... I'm not gonna waste a good clue-by-four on someone with no brain...
- 2003, Willow Polson, The Veil's Edge: Exploring the Boundaries of Magic, p152
- Sometimes the magic "clue-by-four" is so obvious that even your oblivious rock-headed uncle picks up on it.
- 1993, Anonymous, Re: Recent Events from the Horseman, alt.sex.bondage,