Meaning of line in "Line one's pockets" [closed]

Solution 1:

As commented above, to line is to cover the inner surface of something. The illegal activity connotation comes with the sense that the taken money is of an amount that will easily go unnoticed. There's a sense of subtlety. When compared with the idiom "to fill one's pockets" you can get a feel for the difference. Outright theft versus stealthy embezzlement.

Solution 2:

To line something in this context is to give it a lining, defined as "a covering or coating for an inside surface." Typically that lining would be cloth, but in this case it is money. It's a bit of a euphemism, a way to avoid saying the "dirty word" money.