Misunderstanding

Once, when powerbank had yet to be a thing, I didn't know what it's for. My father had one of the earlies powerbanks that time and I thought, underlining the word 'bank,' that it might be a marketing name for external storage device. I also thought that the word 'power' was to emphasize its strength, reliability, or its ability to hold data. Well, it turned out that I was wrong, obviously. I should have known better. Powerbank is a device that hold electric charges for the convenience of having a portable mean of charging other devices, like smartphone. I could have guessed it from the word power. I mean, the term is quite vague and I had not even known the physical form of the device, only the name! You would have guessed that wrong too given the lack of information.