How to use dictionary look up on websites that prevent it?
Assuming I understand your question correctly, you should be able to use the context menu to select the Look Up option.

More specifically:


In your browser double-click on a word to select it
Then right-click (or use ctrl+click) to bring up the context menu (e.g. if you double-click on the word house to select it and then right-click on it, you'll see a context menu such as:





From this menu you can select the top Look Up option and then you'll get the dictionary definition.