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It was originally for "merging" document templates with tables (e.g., mailing list of people's names and postal addresses), for customized paper mailings, mailing labels, etc.

https://en.wikipedia.org/wiki/WordStar#Add-ons_and_companion...

I don't know whether the term predates its use by WordStar and microcomputers. It sounds very batch-processing.

(Source: Had WordStar 3.3 for generic MS-DOS as kid, bought a surplus copy of an Epson QX-10 edition of Mail Merge for $5, somehow got that turned into a copy I could run, for my Alex P. Keaton adolescent business ventures.)

Wow word star with mail-merge seems to have cost almost $500 in 1981...

Indeed. I like the German "Serienbrief" (“mail series”) much better.

the German for word processing textverabeitung probably sounds a bit more natural also.

It can also do printed letters, filling in the physical address at the head of the letter. You can then print out the merged documents.