The spelling is almost as unpleasant as the experience itself. 'Diarrhoea' is one of those words that makes you very grateful that spellcheck facilities exist.
Picture by Hannah Johnston/Getty Images
Our regular oracle on spelling matters for Australians, the Macquarie Dictionary, favours the spelling 'diarrhoea', though it notes 'diarrhea' as an alternative. (It originates from the Greek word 'diarrhoia', meaning "a flowing through", though etymology is rarely a helpful guide to modern spelling or meaning.)
Either way, the key element to remember is the double-R followed by an H. This is difficult precisely because it's not a letter combination we regularly encounter in English, so you have to chalk it up as something you need to memorise despite an apparent lack of logic. One possible trick? Use the mnemonic "really, really hurts" to keep that sequence in your mind.
Incidentally, the definition supplied in the Macquarie also serves as a reminder that dictionary compilers don't always put an emphasis on clarity when it comes to meaning:
An intestinal disorder characterised by morbid frequency and fluidity of faecal evacuations.
The appearance of 'faeces' I can live with, but 'morbid'? It might be clinically accurate, but it doesn't help explain anything.
Lifehacker's Mind Your Language column offers bossy advice on improving your writing.