This is a difficult word to pin down because some people perceive wershness as bitterness and others regard it as unpleasant blandness. Earlier uses seem to be more in line with wersh as a synonym for weak, feeble and lacking in flavour. Ramsay’s Proverbs (1736) tells us that A kiss and a drink of water is but a wersh disjune (breakfast)’. Quite a few of the quotations in the Dictionary of the Scots Language refer to lack of salt. As wersh as saltless kail is an apt simile to demonstrate this sense. Porridge without salt is pretty wersh too. Jamieson illustrates wersh in his 1825 dictionary with: I dinna like them [porridge]; they’re unco werse; gie me a wee pickle saut. Using the word figuratively and alliteratively, the Transactions of the Hawick Archaeological Society (1862) record that The minister had a weary warsle wi’ a wersh discourse and most students have sat through a wersh lecture or two. Wersh weather is raw and damp. Even a kiss might be wersh: A kiss athoot a beard is like an egg athoot saut.
The later sense of bitter or sour does not appear until the early twentieth century and sour is clearly what is intended by Hugh McDiarmid in Sangschaw (1925): Wersh is the vinegar.