They’re the same meaning just about exactly. Maybe lethal is a little more “fancy” if that makes sense. There’s a lot of pairs of words in English like that, where one synonym came from old germanic/norse languages and the other came from old french/latin languages.
That’s why wedding vows say “to have and to hold”, for example. More educated people back in the day would use “have” (from habere in latin) and more common people would use “hold” (idk exactly from where but i assume old german or something.) When there was a wedding they wanted everyone to understand what was being said.
You can definitely say lethal fart and nobody would think it weird.