I think I understand most Latin grammatical terms in relation to what seems to be their etymology in Latin: cases from nominare, accusare, genus, dare, auferre; tempora from praesens, perfectum; verbal forms from finire, particeps, gerere; numbers from singuli, plus. These are just examples; I have surely forgotten many things from my list.
I have trouble understanding why the supine is called supinum. It seems to be related to the adjective supinus, meaning "backwards", "retrograde", "sloping", and similar things. There is also a related verb supinare. Why did the form of the verb get its name from this adjective? What is "backwards" about the supine?
If there is an explanation from ancient or modern grammarians, I would be happy to hear.