Gordon (and MtheGM) - if we can be sure of anything, we can be sure that 'lish' hasn't got anything to do with 'delicious'. Here's the fruit of a few minutes' googling:

Lish adj Fit, agile Musta bin a gae lish coo er a lauw yat. It must have been a very agile cow or a low gate. It is popularly believed that this is the correct reply to the question, 'can a cuddy lowp a yat?' Lore has it that it is used to identify Cumbrians exiled in foreign parts, in a similar manner to the way that Masons identify each other through use of esoteric phrases.

[This is a bit odd, as 'cuddy' means 'donkey', not 'cow'.]

Lish, c., s.w., Leesh, N. supple, active.
Lishlike, G. well-made.

[I think "c." etc are short for "central", "North" and "southwest", i.e. regions of Cumbria; not sure about "G.", though]

Instances of misapprehension of the meanings of unaccustomed words and phrases are often quoted in the district. One of the best of them was told of a wealthy yeoman, well known in West Cumberland, who was sent when a youth to Green Row to finish a not very complete education, and being asked in class what is a verb active, replied, "a lish an."

[Pauses while gales of laughter subside]