To say that someone can’t lie straight in bed, or in the fuller form that they are so crooked they couldn’t lie straight in bed, is to call them thoroughly dishonest - a person who lies out of habit and couldn’t deal straight with you if they tried. It is an insult with a grin on it, usually delivered about rather than to the person concerned, and the target is typically a politician, a salesman, a dodgy tradesman or anyone else whose relationship with the truth is felt to be a distant one.
What’s the origin of the phrase ‘can’t lie straight in bed’?
The phrase is a pun, and quite a dense one. Crooked has carried two senses side by side for most of the history of English: the literal one, bent out of shape, and the figurative one, dishonest, which is recorded from around 1200. Its opposite, straight, has long done double duty as a word for honest, as in ‘as straight as a die’. Stack those up and the joke assembles itself: a person so crooked, in the dishonest sense, is imagined as so bent, in the physical sense, that they cannot lie straight even in bed. The verb lie adds a final wink, since lying is precisely what such a person does best. The shortened form only works because listeners already know the set-up; ‘so crooked he couldn’t lie straight in bed’ is the engine, and ‘can’t lie straight in bed’ is the badge worn afterwards.
Pinning down where and when it was coined is another matter. Modern idiom dictionaries generally file it as Australian - one widely used reference calls it ‘primarily heard in Australia’, another labels it informal Australian and New Zealand usage - and it remains a staple of Australian invective, not least in political argument. It is heard in Britain too, and British speakers tend to treat it as home-grown. No verified first appearance in print has come to light, and searching old newspapers for it is complicated by the fact that the words themselves are much older in their literal sense: nineteenth-century print is full of invalids and cramped lodgers who genuinely couldn’t lie straight in bed. The figurative joke is likely to be a twentieth-century development that circulated in speech long before anyone wrote it down.
It belongs to a small family of hyperboles that make dishonesty physically crooked. American English contributes ‘crooked as a dog’s hind leg’, and British slang built the same metaphor into a single word, bent, its standard term for a corrupt policeman. ‘Can’t lie straight in bed’ is the most economical of the lot: four short words, two puns, and a complete character assassination.