I always interpreted that particular invective to refer to your mother's rugae (which are up her... um... lady-garden) which may have been worn down by overuse, and thus the smoothness that he particularly enjoys. Such people need only little lubricant, which is ironic since they tend to be the ones that get most excited.
One could also interpret it as your mother being someone who applies a lot of oil to their body in order to facilitate lewd acts of uncertain value to those uninterested in those particular fetishes - perhaps the liberal application of actual butter? I suspect you would be the only one to truly know your mother's preferences in that regard.
So yes, it's a difficult one to take[1].
D.
[1] a phrase that unfortunately your mother may have yet to utter.
You Sir/Maam are one sick minded person. I suppose you must know this already yes? Perhaps you relish being perceived as such?
I'd question
my mind being sick; it's your mother being discussed after all, and I don't even know
you let alone her - and certainly not as well as Monkfish appears to. Surely though, what she gets up to in her own private life is of no concern of ours, no matter how many men are involved so long as it's consensual.
I'd drop it, personally, as I doubt she'll thank you for besmirching her name like this.
D.