I found a reference to ‘naysmith’ in a work of science fiction. Is anybody familiar with the term?

I could not find any references to ‘naysmith’ as an actual historical term. Following is an excerpt from the book ‘Horus Rising.”

‘Then it occurs to me, Garviel, that only a weapon which questions its use could be of any value in that role. To be a member of the Mournival, you need to have concerns. You need to have wit, and most certainly you need to have doubts. Do you know what a naysmith is?’

‘No.’

‘In early Terran history, during the dominance of the Sumaturan dynasts, naysmiths were employed by the ruling classes. Their job was to disagree. To question everything. To consider any argument or policy and find fault with it, or articulate the counter position. They were highly valued.’

- Horus Rising, pg. 68

I agree with Cascabel's comment that, from the context, this sounds like a portmanteau of "nay" (meaning "no") and "smith" (possibly in allusion to blacksmith, tinsmith etc., but possibly more in it's figurative use: "A person who fashions something non-material, such as an attribute, one's destiny, etc." [OED, sense b]). In other words, someone good at saying "no" or a Devil's Advocate.

For the record, the OED does not have an entry for naysmith; the closest is their entry for "Naysmyth" which comes from John Naysmith, 1808–90, a Scottish engineer who invented both a steam hammer (or piledriver) in 1839 and a design of telescope c1942. It makes no mention of either being related to a cutler.

From an out-of-universe perspective, the quoted passage doesn't indicate that the reader is supposed to be familiar with a word "naysmith" any more than the reader is supposed to be familiar with "Sumaturan dynasts" (which as far as I can tell are entirely fictional). The passage goes on to explain what the word means, which is exactly what would happen for an invented term.

I disagree with the suggestion that the use of the fictional term "naysmith" in this book must be related to the actually existing surname Naysmith. David Robinson's explanation seems plausible, but I think it's not completely certain: I think the only way to know for sure would be to ask Dan Abnett, the author of the book.

Agree with Michael Harvey, a quick google shows that it is related to cutler. https://en.wikipedia.org/wiki/Naysmith

I also found this: https://www.surnamedb.com/Surname/Naismith

It appears the origin is from "Knifesmith".