It has always struck me as rather odd that when English copies a word from another language, it is called borrowing. Borrowing? Isn’t it more like stealing? Do we ever give any of them back? Generally, we keep them and convert them into words we are more comfortable with, as we turned Malay ketchap “fish sauce” into catsup, Indonesian amok into amuck, and French quelque chose “something” into kickshaw.
The funny thing is, we do return some of the words we borrow, though seldom within a reasonable time. Casino, for example, is the diminutive of Italian casa “house” and was used by the Italians to refer to a vacation cottage and later to a dance club. We borrowed it, gave it the current meaning, then lent that word back to Italian as casinò with the new meaning. (Aren’t we generous?)
More often, however, other languages return words to us that they have borrowed. Old French borrowed brown from Old English, rendering it brun. A person with brown hair was soon dubbed a brunet (or brunette if they were a girl). English, of course, then borrowed these words back from French later one.
Crawfish began its life as krabba “crab” in one of English’s ancient Germanic ancestors. It was borrowed by Old French as and went on to become crevis “crayfish”. French returned this word as crevisse which English promptly converted to a more palatable crayfish. Now, since crayfish are fish that crawl, it ultimately became crawfish in some regions. (Changing a borrowed word into something more recognizable is known as ‘folk etymology’.)
French borrowed ward from us but didn’t have a W sound, so they used th closest sound they had: GU [gw]. French then returned that word to us as guard.
So, I guess, borrowing is the right expression for this process, so long as we keep in mind that words borrowed are seldom returned and never returned in the same condition as they were borrowed in.