Username Bug

So, I have been searching for some good 3 name usernames in namemc.com and in 3name.xyz. I have found this username called Fud and I liked it since it sounded good. I decided to change my username to Fud but it didn't work, it said username taken. But, it clearly shows it is not taken in NameMC and in 3name. I am not sure if Fud is a bad/inappropriate word. I even searched it up and it doesn't seem to be a bad word. If I am asking something which has already been asked, please let me know.