When looking up electronics shields the only useful information I could find pertained to Electromagnetic shielding. But when inspecting Arduino shield schematics most offer no protection to the Arduino.
So it begs the question, why are Arduino peripherals called shields?
https://en.m.wikipedia.org/wiki/Electromagnetic_shielding
http://m.instructables.com/id/ATtiny-Programming-Shield-for-Arduino-1/
I doesn't look like they've registered a trademark for it ("shield").
The mostly obvious explanation is the mechanical position of the daughterboard. One book says:
These boards are called “shields,” because they usually fit over the top of Arduino like a protecting shield.
The setup is by no means new. I had an ancient (1990-era) video card that had a 2MB addon-VRAM card (for a total of 4MB); it looked like the image below
The standard name for a subsidiary printed circuit board (PCB) that plugs into a main PCB is "daughterboard"
However, the somewhat artsy communities that produce these small experimental microcontroller boards each tend to invent their own name for daughterboards that have the required dimensions and connector to mate with the primary form of main PCB in that community
Arduino: shield.
Raspberry pi: hat.
Beaglebone: cape.
So they tend to borrow the name of some type of accoutrement that a human might loosely attach to themselves. There is a vaguely protective theme here but I think that is largely irrelevant.
I'm not sure what the first Arduino shield was but it was probably something that mediated between the Arduino and the outside world. Also from its shape it isn't too dissimilar to a rectangular shield, I suppose it also (uselessly) shields the Arduino from light (and fingers). Most Arduinos have no (or little) electrical protection on their inputs and outputs, at least some shields provide a modicum of additional protection - although that is usually not their main purpose.
It's hard to answer questions "why somebody did something non-technical in a certain way". I can think of 2 possibilities, though.
Recently, I was reading an application note written by a large company. When they had to mention Arduino extension board, they wrote "Arduino extension board". [I can't seem to find that app note again. It was by Texas Instruments, iirc.]
Finally, the question has been already answered here.
(trivia: Arduinos are named after Bar di Re Arduino pub, where the primordial ardweenies were meeting up. Source.)
It's a brand name. Compare it to the names that other manufacturers of other systems give to their plug-on, stackable, add-ons.
Why did they choose that brand name? It had to be different from the others. And easy to say. And it helps if it conjures up something small and single-layer. And has good rather than bad connotations.
In the case of Raspberry Pi, HAT is actually an acronym
for Hardware Attached on Top.
Source: https://github.com/raspberrypi/hats/blob/master/README.md
I don't know if the CAPE term used by Beaglebone is an acronym. The same goes for Shield for Arduino daughterboards. Mezzanine modules or daughterboards as a concept have been around for a long time. See the wikipedia article at https://en.wikipedia.org/wiki/Expansion_card.
Consider the PMC (PCI Mezzanine Card) acronym for IEEE Std 1386-2001 daughterboard modules used with VME bus and CompactPCI carrier boards for telecom and COTS military applications (https://ieeexplore.ieee.org/document/944007). Or take a look at other more recent Computer-On-Module (COM) form factors like M.2 (https://en.wikipedia.org/wiki/M.2) or COM-HPC (https://www.picmg.org/openstandards/com-hpc/).
There has been some discussion on the web discussing the desire for standardization for daughterboard and SBC connectivity. See https://hackaday.com/2022/10/05/the-state-of-the-sbc-interface-ecosystem-is-it-time-to-design-a-standard/
Another good overview of the plethora of daughterboard form factors is the GumStix article at https://www.electronicdesign.com/industrial-automation/article/21804810/electronic-design-embedded-design-build-buy-or-both.
Part of the problem in devising a common standard I think is that different form factors have different goals. For many recent Arm-based SOMs (System-On-Modules), the mezzanine module requires expensive high density, high pin count connectors (like Samtec or SoDIMM connectors) for connecting the SOM compute module to a carrier board for a custom I/O. But for other applications, 0.1" spacing on daughterboard connectors is cheaper and also more suitable for easy prototyping / breadboarding. I'd personally like to see movement towards proper standards though. I don't see any commercial advantages to having a closed proprietary ecosystem for boards.
