The dictionaries in Notepad++ are only meant for doing spellchecking or
autocomplete, not for searching out the definition of words,
and are meant more for programming languages than for English
(or other languages).
For this functionality you need to use other third-party products, with the
additional bonus that they will work in all of Windows, not just in Notepad++.
They will either work by selecting the word and clicking an icon
in the traybar,
or by adding a right-click context menu entry to the active window.
For a list of such products with reviews, see the article
Best Free Dictionary and Thesaurus Utility.
Read the article and the comments below it.
One free product that is not in the article is
TheFreeDictionary.