Was the term "shield" in use [for daughterboards] before arduino became popular?
No.

The whole "shield" thing came about at about the exact time as the arduino became popular.

"Shield"s were, are, and will still be daughterboards or modules in pretty much any serious context for the foreseeable future.



&lt;&lt; snark>>
Speaking loosely, "shield" means "extension module I don't really understand (because electronics are just like lego, right?)"
&lt;&lt; /snark>>  

I do make judgments about the knowledge of people who refer to plug-in modules as a "shield", though.