lynnski wrote:Perfectly reasonable? Where do you come from then? 'how's that' is totally different from 'why's that', the two are not interchangeable. 'How is that possible?' is a totally different question from 'why is that possible?'.
As Dave points out, it's quite common in Scotland. Lanarkshire boy, myself. Used it colloquially for many years without ever noticing it was "wrong". The intention of the word does not change if used singularly as a question ("How?", to mean "How come?"), though I'd never write "How?" by itself. I'd assumed it was just another peculiarity between Scottish dialects and formal written English.