Which plural do you use for a word that should have a regular plural but ends with a word that has its own irregular one? The example that made me ask was "leatherman" (the multitool) but there are plenty of others.
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Short answer: it's up to the company, but nobody cares what the company says.
The official plural of Walkman according to Sony is Walkman Personal Stereos, not *Walkmans or *Walkmen. People widely use walkmans nonetheless.
Since Leatherman refers to a brand name and not a specific product, there probably is no official plural—even though Leatherman is used quite often colloquially to refer to a Leatherman-brand multitool. But people say Leathermans nonetheless. Seeing a pattern?
One more amusing example: Bluetooth is used colloquially on its own to refer to a Bluetooth phone headset; a cursory Googling shows Bluetooths dominated by Bluetooth-related results, while Blueteeth gives mixed results, including a number of shoddily edited product sites, one of which claimed that the “pecifications” of a phone include “blueteeth”.
So that's something.
In this case the tool is named after a Mr Leatherman - so it's a 'leatherman tool' (sing) or 'leatherman tools' (plural).
The general rule is- If the compound word is a type of the base noun, pluralize as the irregular. Otherwise pluralize the compoud as a regular noun. Fireman becomes firemen Walkman would become Walkmans Leatherman would become leathermans. Of course trademark names are not subject to the usual rules.