I found a dictionary here in the Books forum:
GNU Collaborative International Dictionary of English, V1, 9, July 2007
https://www.mobileread.com/forums/showthread.php?t=12000
I simply copied it into eBooks directory. It seems to contain a word definition per page, about 20K words.
Then I went into a TXT file and tried lookup command. It jumped into the dictionary doc at the word definition. Then I used Back command to return to the orginal doc.
I am not sure if it handles suffixes, like finding 'house' when you select 'houses' or 'housed'. Also, the parser seems to be rather simplistic and can include commas or periods in word selection. Such as:
"... at the house."
You can select "at", "the" and "house."
Of course, "house." lookup fails.
Anyway it is a start and a very nice feature to have. As you already know it is not available for PDF files (I guess they cannot parse text in PDFs yet) - another annoyance. Again, we'll just have to wait for another firmware.