What's that when it's at home

I am interested to learn of the derivation of the phrase "what's that when it's at home", where 'that' can be a word that the speaker/writer does not understand the meaning of.

with thanks
Kathy

The 'that' could also be an event or object that is unexplained or unusual, e.g. a strange noise in the night or a bizarre haircut.

The phrase was mostly used in the English Midlands and North and probably originated there. It is rather archaic now and I wouldn't expect to hear it from anyone under sixty.