I think I first heard this question on a television quiz show – I can't remember which one – when my memory tells me that these were claimed to be the only words in the English language that end in these four letters. Some years later I asked that question in a charity quiz for participants in Stockport Quiz League, and the questions guru came up with a list of words that was as long as your arm.
Most of them seemed to be technical words, used in chemistry or biology; but fair enough, they're still words. He had me bang to rights.
I tried a Google search, and found a page on infoplease.com that listed 14 words – including my four, but also words like 'apodous' (having no feet – a synonym for the more common 'apodal') and 'molybdous' (concerning or containing molybdenum). This was actually in answer to someone who asked "There are four words that end in 'dous' in the English language and I can only come up with three – stupendous, tremendous and horrendous. Can you help me find the fourth?"
My Google search came up with enough hits to indicate that "the four words that end in 'dous'" is a thing. One website called wordexample.com lists 83 words – including 'biohazardous', 'ultrahazardous', and many more of the technical kind. I wonder if this is where the Stockport man got the list he hit me with?
Anyway ... the Stockport incident was a lesson to me, and a reminder of the need to be careful when using phrases like "the only four words". But I still like my original question, and I've kept it on the website with a less definite wording. If asking it again, I'll make sure I'm armed with a dictionary to check out any more obscure words that contestants might come up with!
© Haydn Thompson 2020