Russian is magical. You can mix a simple word with a bunch of even simpler little words, pass your magic wand over the mixture and get the linguistic equivalent of a flock of doves. You can watch the magician's hands — that is, the grammatical structure of the language — but you can't quite catch the moment when the magical transformation takes place.
Take the simple word мало (few, little, not many). На концерте было мало людей (Not many people came to the concert). Nothing up my sleeves!
But when you add того (of that) to мало and pass your linguistic wand over the phrase — presto! You get the new meaning of moreover, in addition to, on top of that. For example, here's how a woman used the phrase in Anton Chekhov's play "The Bear": … я его любила и была ему верна. Мало того, он умер, и я всё ещё верна ему. (… I loved him and was faithful to him. And what's more, even after he died I remained faithful to him.)
Then add что (that) to the word mix in the magic hat and — abracadabra — you get мало того, что … (it's not enough that … ) For example, here's what a decidedly more modern woman had to say about her man: Мало того, что мужик на шее сидит, так ещё, сволочь, удовольствие получает! (It's not bad enough that the guy lives off me; the sleazeball is enjoying himself!)
Now my assistant will drop мало with ли into the magic hat along with the pronoun of her choice. Shazam! You get a white rabbit, three doves and a bouquet of roses — that is, a phrase that has several meanings depending on the context. If said in irritation, the phrase мало ли can mean "it doesn't matter."
For example, if we continue to follow the trials of our stereotypical unhappy couple, when the wife questions her feckless husband about his dining companions, he might answer: Мало ли с кем ходил в ресторан! (What does it matter who I went to dinner with?)
In other contexts, мало ли might stress the great number or variety of something: Мало ли что говорят! (People say all kinds of things!) Мало ли где я её встречал! (I could have met her anywhere!)
Put a noun in the genitive case into the magic hat with мало ли and you get the opposite of a few; you get lots of, many or plenty. Мало ли деловых приёмов, где я мог бы её встретить! (There have been plenty of corporate receptions where I might have seen her!)
All alone as an exclamation, мало ли что has the sense of "so what." Our grossly stereotypical husband might shout: Ну, ходил в ресторан. Ну, выпивал. Мало ли что! (Yeah, I went to a restaurant. Yeah, I had a drink. So friggin' what!?)
And for our grand finale, let's drop мало with не покажется (will not seem) into the black hat. What goes in as "it won't seem like much" is magically transformed into something "so big and awful you're going to be sorry it ever happened." Мало не покажется is a threat, sometimes a joke threat, sometimes not.
Wife to husband, getting in the last word: Если я узнаю, что ты был с ней в ресторане вдвоём, мало не покажется! (If I find out you were alone with her in the restaurant, you won't know what hit you!)