RadCaptcha has an internal list with offensive words to exclude by default. The mentioned word is included inside that list as well. I have also tested the captcha's internal methods and they properly exclude that word.
Could you please send us the exact captcha's declaration, so that I can make a test with it? Can you also tell us which Telerik UI version you are using?
Regarding enriching the list with the offensive words to exclude this is not possible with the current implementation of the control. You can, however, use our feedback portal
to log this idea for the control. As more and more clients vote on the item its priority for implementation raises.
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items