In general, the prefix 'un-' expresses negation when it is attached to a noun. In fact, 'un-' "is now the [prefix] which can be used with the greatest freedom in new formations." (Oxford English Dictionary or OED). So this is true for the word 'unappreciation'. It means 'no appreciation.' You can also think of it as "without appreciation' or 'lacking appreciation'. There is also the adjective 'unappreciative' which means the opposite of 'appreciative'. http://www.thefreedictionary.com/unappreciative However, the word 'unappreciation' is rare (the OED has only example). Most dictionaries will not have it. A little later in the same text, there is a heading 'Unappreciation,' so you can read more about how the author applies the word to Shakespeare. In short, "un-' means negation when attached to a noun. When attached to a verb, it can mean 'opposite', as in 'undo' and many other verbs.