It's not really that limited. Basically it just needs to be a list of words in any format really. An online tool like wiktionary wouldn't really work as it needs to check so many different letter combinations if they're a word each placement (6 checks in the most simple case of placing two 2x2 blocks next to each other) so it would become slow.
Yeah I like the definitions too. I remember testing out the game while I was originally making it seeing some unexpected word being made thinking "is that even a word" and that gave me the idea. Learning new stuff is great and if I see an opportunity to offer such in my games, I always take it. And that's why I show random definitions of the word too. Even if you know what you're using, you might still learn some new use case for the word. Even though as a flip side it seems to confuse some people to think it's displaying a "wrong" definition even though every obscure one is is pretty as legit as the common intended one.
Obviously having definitions for everything would be optimal but sadly it's not quite feasible. I'm just using a single source for them (https://dictionaryapi.dev/) and even though it has like all the "normal" words, it doesn't have everything. I don't think any source would. Even adding something like wikipedia/wiktionary wouldn't probably cover everything and would be a lot more difficult to fetch and format too. And of course AI could be of help too but would get a lot more complex to implement for such a small feature.