https://www.mobileread.com/forums/sho...d.php?t=273820
you can add however many dictionaries you would like using the above method, so long as those dictionaries work on Kobo. There is a link in the thread I linked to which provides alternative dictionaries, and there are additional English ones there.
You'll need to patch your Kobo, information on that can be found here:
https://www.mobileread.com/forums/sho...d.php?t=269574 presuming you are on the latest firmware. If you are not on the latest, then you will need to either update your device, or find the thread for patching the version of firmware you are using.
However, as has been noted and shown by others, the word is in the built in dictionary.