Q. I have scoured the internet looking for an answer: How are plurale tantum [plural only] words like “pants,” “scissors,” “sunglasses,” and “manners” constructed using the suffix “-less”? Would it be “pantless” or “pantsless,” “scissorless” or “scissorsless,” and so on? I can find arguments for either construction for each term. I’m hoping there’s a grammar rule (somewhere) that will guide me toward a definitive answer. If the “s” is retained before the suffix, most words become awkward to say the least (“trousersless,” “slacksless,” “shearsless,” etc.). Is it simply arbitrary? Based on popular usage? Something else?
A. We can’t cite a rule, but we know that the suffix “-less” almost always gets added to the singular form of a noun. And we could guess that this fact would influence how “-less” might be added to a word like “pants.”
In other words, if “shoeless” (no shoes), “witless” (no wits), “childless” (no children), and “toothless” (no teeth) are all standard, then “pantless” (no pants) would seem like a reasonable option. And that’s what the OED has (“pantless,” not “pantsless”).
But compared with the standard singular forms “shoe,” “wit,” and so on, “pant” as a singular noun is kind of rare. You might consider ignoring the OED in this case and going with “pantsless” instead.
Some of the other words you mention may work better without the “s.” “Scissorless” and “trouserless” seem OK, maybe because the adjectives of those words are commonly spelled without an “s,” as in scissor kick* and trouser pocket (the adjective form of “pants” more often retains the “s”: pants pocket). And “mannerless” is in Merriam-Webster and the OED.
“Sunglassless” and “sunglassesless,” however, both look like mistakes, and “slack(s)less” and “shear(s)less” also have problems (starting with the fact that slack and shear have multiple meanings). Rewriting would be best (e.g., without sunglasses, slacks, or shears). Or try a hyphen (e.g., “sunglasses-less”).
In sum, approach these terms on a case-by-case basis, and don’t settle for an awkward or ambiguous result.
* Merriam-Webster records only “scissors kick” (as of October 2024), but that spelling has become less common than “scissor kick” in published books in recent years, as this n-gram from Google suggests.