In The Western Star Chief Inspector Japp replies to Poirot “As true as I’m riding this bicycle”. I was so nonplussed by the expression, that I didn’t even get the context of it; I do believe he was making a point of it not being true, because when Poirot looked at him incredulously, Japp showed with his hands there not being a bicyclce.
What is the origin of that expression, and what does it actually mean?
- Lynn Connolly, 2008: Close Encounters … of the Shirley Kind, p. 59.
- Deric Barry, 2014: Innocent on the Run, p. 74
- Bonkworld: Howon Howay, 21.9.2001.
- John Breeds Magic: “Funny Talk! For Kids’ Magicians”, undated.
Best Answer
TL;DR: It's a common twist on a different common idiom; in this form, it means "What you just said is clearly false."
As sure/true as I'm [fill in the blank of what I'm doing or am] is a common idiom used to mean that something is definitely true.
The blank can be filled in lots of different ways; [as sure] as I'm alive is the earliest form I can find, for example in this ostensibly 17th century work (snippet view), this "Epilogue" from 1756, and this English-Swedish dictionary from 1757.
The most common formulation today might be the (usually American English) [as] sure as I'm standing here (M-W) but it is often shifted for comical effect. For example:
So long as whatever fills in the blank is obviously true, the original meaning of the idiom remains.
However, if the end of the phrase is patently untrue then the meaning of the idiom is flipped to mean the rest of what I'm talking about is also false.
Some of these "flipped" versions have passed into semi-idiomatic use, such as sure as I'm a foot high (apparently Irish slang) and Inspector Japp's statement, used when one is obviously not riding a bicycle.
While a bicycle or bike is the most common thing (not) being ridden, there are other variants of the form true/sure as I'm riding this [thing I am obviously not riding], such as:
Most of the above appear to be UK sources, but sure as I'm riding this bike has also apparently made it at least to New Zealand.