Legal dictionaries are useful for looking up legal terms of art, but they are not binding on courts. After all, anyone can publish one.
In the US at least, courts prefer to use the common meaning of words whenever it's reasonable to do so. If the common meaning doesn't make sense in context, they won't simply fall back on a legal definition, but will look for evidence of what the parties actually intended the words to mean.
In the case of disparagement, several courts have held that the common meaning of "disparage" is what applies to non-disparagement clauses.