I have never used this phrase in conversation in my life, nor have I ever heard anyone say it directly to me in conversation. I have only infrequently come across the phrase while watching television shows or movies, or while reading books.
I have even less experience with it than GreenWhiteBlue. I guess I must have come across it, as it is still occasionally to be found, but it seems to have escaped my attention all these years (it first came into use somewhere around the 15c.).
The OED has a quotation from 1991: Though he claimed never to have read a book, he was highly intelligent and full of mother wit. [Daily Telegraph, London]
I am not at all familiar with the expression 'mother wit', although it rings a very faint bell.
I suppose it means something like 'common sense' or 'the wits you were born with'.
I suppose I would immediately work out, almost translate, what was meant in a clear context but I wouldn't think of using it, since it is hardly in even my passive vocabulary