The phrase lump it means to dislike and have to put up with (something disagreeable). The full idiom is: "If you don't like it, lump it." In other words, even if you don't like something, you have to bear with it, because the situation will not change. In your case, did you have to dig around for a tissue in place of the never-to-appear napkin?
At a restaurant famous for its rude servers, a waitress told me to lump it" when I asked for another napkin. Can you tell me about that phrase?"
Here, in The Adventures of Tom Sawyer, Tom asserts his own turf in the presence of Alfred Temple, a boy who is new to Tom's shabby village. Alfred is well-dressed — for a weekday — and is even wearing a "dainty" hat:
"What's your name?"
"'Tisn't any of your business, maybe."
"Well I 'low I'll MAKE it my business."
"Well why don't you?"
"If you say much, I will."
"Much - much - MUCH. There now." . . .
"Smarty! You think you're SOME, now, DON'T you? Oh, what a hat!"
"You can lump that hat if you don't like it. I dare you to knock it off — and anybody that'll take a dare will suck eggs."