it simply means to make someone a bastard.
"why did I have to bastardise our new son?!?!?!"
To make a bastard doesn't necessarily have to refer to people. Giving a legitimate word a new meaning could be bastardising the word. Basically, it's butchering the English language. See orphaned
. If you have an idea, and someone takes the idea, changes it around and gives it new meaning, that's bastardising. If you design a newsletter, for instance, and someone else takes over and keeps part of your design but applies elements that don't match up with yours, that's bastardising. You get the drift.
Henry, how do you like the newsletter now that Sue's been doing it for a year?
I'd like it better if she hadn't bastardised my design.
Well, how do you like urbandictionary.com?
Half the people putting stuff up there just bastardise the language.
To Utterely and Irreversably fuck it up in the way only a child born out of wedlock could ever do.
I was having an intimate moment with the maid when Bob the Bastard entered with my wife and totally bastardised my night.