"Een jas is geen ondergoed."
Translation:A coat is not underwear.
Underwear is accepted (see above). I assume the report from around the same time you posted this is yours and you used is not an underwear.
Underwear is a mass noun and uncountable and generally you don't use an indefinite article when in combination with these.
When the Dutch say "ondergoed," do they mean a specific article of clothing, such as panties or a bra, or do they mean any underclothes? I put "underclothes," since in my locality "underwear" is specific to panties/underpants/briefs, and doesn't usually include other things such as bras and undershirts (it can, but that's not what people generally think you mean), so "underclothes" seemed to me to be a better English translation, at least for my area. If the Dutch word "ondergoed" encompasses several kinds of underwear, then I think "underclothes" should also be accepted as a translation.
To you it might be, but if someone is talking about underwear I'll assume they're talking about what you wear underneath your pants, rather than a bra or undershirt.