This is a west country/rural word of old. It means a burning smell or a smell so bad you think you can see it.
My mother, having been born on the Mendips in 1932 uses it alot.
The grilled kippers left a terrible smeech in the kitchen.
The coal fire created a foggy smeech in the lounge.
My late mother who came from Cheltenham used 'smeech' to mean 'over-heating fat', i.e. a combination of 'smoke' and 'steam' so my brother and I still thought it was only used in our family as we'd never come across anyone who knew it. but now we know better.
When I arrived at a pancake party I exclaimed 'What a smeech and everyonesaid 'What?''