Similarly to "isch" from German, which we filed the serial numbers off of, and respelled "ish". For some reason we are squeamish about doing that to French.

Perhaps ironically, we took squeamish from French escoymous, but substituted "our" suffix.

"From Middle English -ish, -isch, from Old English -isċ (“-ish”, suffix), from Proto-Germanic -iskaz (“-ish”), from Proto-Indo-European -iskos."

https://en.wiktionary.org/wiki/-ish

Yeah, it's a little misleading to say that English borrowed that ending from German, English is a Germanic language and always had it.

Ok, and "-ish" and "-esque" clearly both trace back to the Proto-indo-european. So we're really just talking about spelling standards, which are a recent innovation.