I was wondering about that myself, actually, so I did a quick search and turned up this site:While you’re here, does Pokān actually have a translation?
Translation of ポカーン from Japanese into English
English translation of ポカーン - Translations, examples and discussions from LingQ.
www.lingq.com
It gives the translation as "vacantly; blankly; listlessly; absent-mindedly". It sounds more like an onomatopoeic word to me, though, so I kept looking and found this:
ぽかん, ポカン, ぱかん, パカン, pokan, pokan, pakan, pakan - Nihongo Master
vacantly,blankly,absentmindedly,openmouthed,with one's mouth wide-open - Meaning of ぽかん, ポカン, ぱかん, パカン, pokan, pokan, pakan, pakan. See complete explanation and more examples and pronunciation.
www.nihongomaster.com
That one uses a short a sound instead of the long ā, but it adds "openmouthed; with one's mouth wide-open; gaping; flabbergasted" to the list of definitions, as well as the onomatopoeic "with a whack; with a thump", so... take your pick? You'd be a better judge than me, having seen the show.
I wanted to ask, actually: how did you first come across it anyway?
And lastly, also from your post...
Out of its original context, that sounds like the kind of thing that would be very disappointing to hear in the wrong context.Neil, please, stop, it hurts. I can’t take any more