Okay, I'm not the sort of person to blame others for my mistakes (what am I talking about? I'm exactly that sort of person!) but look at this sentence on Big Dave's website, doesn't it look for all the world that the 3 letter gentleman is" inside" another word meaning alarm? That, coupled with Ifor's reference to abbreviations not in Chambers and I went down an Alice in Wonderlandesque rabbit hole ....
"reverse, the ‘gentleman’ within a word denoting an ‘alarm’
Actually, now I look at it again, it does mean exactly what it's meant to - but I'd reached the end of reason..!
Many thanks Malone - you may have just saved my sanity - what's left of it 😜