The only other usage than "horsing around" I can find on a (SafeSearch) Google, is the expression "to horse a gentlewoman", which I assume is an historical expression meaning the general assistance of a lady in enabling her to ride out. Then again, maybe I'm being naïve and it has a more Catherine the Great connotation.

I believe it involves a carpenter's sawhorse (suitably reinforced and stabilised), a willing 'victim', a quantity of rope and a good imagination. It might be a good idea to keep a chiropracter on the speed dialer too IMHO.

Earlier today something similar to Projoy's comment on "thicken" occurred to me, although I noted that "enlarge" seems to apply the suffix as a prefix. There are words such as "enliven" which seem to follow the same pattern, except "liven" isn't an adjective (at least now). Anyone know a reason why "enlarge" behaves like that?

[Darren] No idea, but it did make me think of the verb from "bold" is "to embolden", which has both suffix and prefix (though not necessarily in that order).

(Darren) "Enlarge" is from the French enlarger and may just be a copy, so to speak. A more regular formation, reading between the lines of my 1964 COD, would be "largen", (cf thicken) the prefix en- + adjective being rare. En- + noun or verb is common though and explains "enliven" either as "to give life to" or to (intensively) liven.