I read through a paper titled “Aquinas’s Parasitic Cosmological Argument.” (Medieval Philosophy and Theology 1 (1991): 119-155), by Scott McDonald. On page 120, he says:

I will draw on both of Aquinas's statements of the proof, the "first way" found in ST 1.2.3 and the first of the two "Aristotelian" proofs found in Summa contra gentiles (SCG) 1.13. The SCG and ST statements of the proof differ in two significant respects. . . .The second respect in which the two presentations of the proof from motion differ is in the statement of the conclusion. The SCG version of the proof concludes: "Therefore it is necessary to suppose that there is some primary unmovable mover" (primum movens immobile), while the conclusion of the proof from ST is apparently weaker: "Therefore it is necessary to arrive at some primary mover that is not moved by anything" (primum movens quod a nullo movetur). I think this second difference is important and I discuss it in the seventh section, below.

Now, my translation of SCG consistently renders primum movens immobile as "some prime unmoved mover." There is, however, obviously a stark difference in that and "some primary unmovable mover."

So which is it? Should immobile be rendered "unmoved" or "unmovable"? Google Translate suggests "unmoved mover," but taken alone, it suggests "immovable."

Help?!?

Lewis & Short gives this entry:

immōbĭlis (inm-), e, adj. in-mobilis,
immovable.
Lit.
In gen. (class.): terra immobilis manens, Cic. Rep. 6, 18: elephas tardum et paene immobile animal, Curt. 8, 14: balaenae ad flexum, Plin. 9, 6, 5, § 13: rigor, Quint. 9, 4, 101: immobilior scopulis, Ov. M. 13, 801.—
In partic.: res, immovable property, real estate, Dig. 2, 8, 15; 41, 3, 23; opp. res mobiles, Ulp. Fragm. 19, 6, 8.—

Trop., immovable, unmoved, unalterable (mostly post-Aug.): ardet inexcita Ausonia atque immobilis ante, Verg. A. 7, 623: donec princeps immobilem se precibus et invidiae juxta ostendit, Tac. A. 16, 10: adversum plausum ac lasciviam insultantis vulgi immobiles, id. H. 4, 2: isti apathiae sectatores, qui videri se esse tranquillos et intrepidos et immobiles volunt, Gell. 19, 12, 10: statua pro rostris cum hac inscriptione: PIETATIS IMMOBILIS ERGA PRINCIPEM, Suet. Vit. 3: omnia, quae mensurā continentur, certa et immobilia congruere sibi debent, Front. Aquaed. 34.

So, to answer your question, immovable or unmoved, well, yes! (As Lady Bracknell would put it "both if necessary, I presume!")

Hmm. See, Collins has "immovable" as a gloss, too, but of course, it's a classical dictionary. So both are options? Any ideas on which Thomas actually intended? In SCG 1.13.3, he puts in outline form the argument from motion, from which he draws the conclusion that there must be primum movens immobile. The Latin:

Quarum prima talis est: omne quod movetur, ab alio movetur. Patet autem sensu aliquid moveri, utputa solem. Ergo alio movente movetur. Aut ergo illud movens movetur, aut non. Si non movetur, ergo habemus propositum, quod necesse est ponere aliquod movens immobile. Et hoc dicimus Deum. Si autem movetur, ergo ab alio movente movetur. Aut ergo est procedere in infinitum: aut est devenire ad aliquod movens immobile. Sed non est procedere in infinitum. Ergo necesse est ponere aliquod primum movens immobile.

Later, we see him consider the objection that perhaps the argument does not terminate in an absolute first mover, but one of many mundane first movers (human souls, for instance). Here's the opening paragraph of argument by which he concludes there must be a true first mover (1.13.21):

Quia vero, hoc habito quod sit primum movens quod non movetur ab alio exteriori, non sequitur quod sit penitus immobile, ideo ulterius procedit Aristoteles, dicendo quod hoc potest esse dupliciter. Uno modo, ita quod illud primum sit penitus immobile. Quo posito, habetur propositum: scilicet, quod sit aliquod primum movens immobile. Alio modo, quod illud primum moveatur a seipso. Et hoc videtur probabile: quia quod est per se, semper est prius eo quod est per aliud; unde et in motis primum motum rationabile est per seipsum moveri, non ab alio.

What is interesting here is the addition of penitus. So we seem to have the idea of not being moved at all . . . or can that also mean absolutely unmovable?

These are different propositions, it seems to me. Something totally unmoved may or may not be absolutely unmovable. Right?

Or wrong? Whatever the idea is, it seems we should get it right, because it is pretty important to Thomas here . . .

"Unmoved" is philosophically the more important point; "immovable" proceeds from it. God is not unmoved because He is immovable. He is immovable because He is the unmoved source of all motion.

True. And that has the added benefit of being the "weaker" statement. To say the First Cause is unmoved is "easier" to prove than He is "unmovable," at least insofar as, like you say, immobility proceeds from being unmoved. So all I really have to show is that, contra McDonald, the First Way does conclude in an absolutely unmoved mover, which is easy enough.

Much appreciated.