"like it or lump it!"

This means that something that you will not like is going to happen, but you just have to accept it - there is no choice. Basically the same as "take it or leave it".

But where did this meaning of lump come from?

Usually, the word 'lump' is used as a noun - an indistinct mass of something:

a lump of dough

a lump in a sauce (grumo)

But also:

 "a lump in your throat" (un nodo alla gola)

How many lumps of sugar do you want in your tea? (cubetto di zucchero)

It is less common to use "lump" as a verb - though the most common use is now "to lump together" - to put a group of things together in no particular order

However, the meaning here refers to a common use in the 16th century which was:

to lump - to look unhappy or disagreeable ...hence the opposite of to show that you like something

Click here to see the idiom in use.