So, there we were last weekend slogging through furniture stores. As my mind glazed over, I thought about the different words for seating. (I also was watching for lions, tigers, monkeys, etc. at that one furniture store. None spotted, not even a leopard.)
A couch is armless like the “fainting couches” used in Victorian times and by some of us when we look at our investment statement these days. Those who know French say it comes from a word derivation that means to lie down.
Over time, arms have been added, and in some cases, the legs removed to evolve into what we now call a sofa. The term sofa seems to come from an Arabic word that means a bench or something else to sit or recline on but not for laying. Typically, they have a back too, the furniture not the Arabs.