'zackly.
My answer would be totally straightforward: Well, mom, it’s a Japanese word that describes many men ejaculating into the face of a single woman.
And then if she asked you what “ejaculating” means …?
It’s a Japanese word that means something like “splash”.
Okay, it’s time for me to admit a deep, dark secret.
I was once the parent in this exact same situation.
My daughter was showing off her knowledge of Japanese anime, and somewhere in the middle of her explanations of various styles and terms, my questions went from “why do they all have big eyes?” to “and what’s the thing called bukkake?”
By the look of absolute horror on her face, I cleverly figured out that the word really didn’t have anything to do with the topic at hand, and I said, "Or is that something entirely different?’
The important thing to remember is WE NEVER SPOKE OF IT AGAIN!
So … what, you just pantomimed it?
We need a “like” button.
a Finnish pastry
it sure is now -
no, you finish pasty.
Did I somehow give the impression that my mother was mentally challenged?:dubious:
Actually, both my parents were always completely frank and open with me about sex, although they overall points of view differed. they were the first people I told when I lost my virginity, and I distinctly recall a lunch with my father during which he was making illustrations on a napkin of how it was absolutely possible to bring any woman to orgasm with a penis. I pooh poohed him, but it turned out he was right. 
And THIS is why my becoming a pornographer was practically inevitable. 
Charles Bukkake? A Los Angeles poet/novelist who dabbled in comic books. Not to be confused with that other Charles Bukkake who is a dirty writer.
No, you did not give the impression that your mother was mentally challenged.
I posted this years ago, I think it would be a good response if your mom knows about the sea documentaries of Jacques Cousteau:
Do you remember seeing a documentary once that showed how sponges (or was it urchins?) were filmed in the act of releasing their seed and eggs? There was so much that a white milky cloud actually covered the sea floor! That is because those critters reproduce at the same time. Of course the narrator then was just describing the scene as a matter of fact, but I always picture Jacques Cousteau as the narrator looking at that and saying:
‘And soo we have ze marine creatures all wallowing in ze Bukkake… pervers!!’ 
Heh. I was was watching outtakes of “Clerks 2” on youtube the other day, and my 76-year-old mother walked in and repeated something Jason Mewes had just said.
“Poopie Trim?”
I just explained it away as a part of the silliness of the outtakes. I’m sure she had no idea what the actual words were that they had said - the words, out of context, make little sense.
In older days, I probably would have cleaned up the mess from the spit-take I had made upon hearing the question (IIIII-RONY!), asked her where the hell she had heard that, then, carefully and clinically, answered her “bukakke” question. She’s got dementia now, so I think I would just avoid the conversation, just as I avoided explaining, “poopie trim.”
Comedy! Nyuck nyuck!
I believe Insanity Wolf would say, if your mother asked…
DEMONSTRATE
My incredibly straight laced mother would absolutely die if given even a clinical explanation, so it would have to be “Mom, you don’t want to know.”
I’m not sure that’s true.
She didn’t say it had to be a human penis.
It probably isn’t, but it’s much, much closer to true than I would ever have thought. (and I’m not saying straight in-n-out will do the trick, there’s definitely technique involved…)