This is an old biblical idiom but still used. It refers to the lees (dregs, sediments) of wine or other liquids that settle in the bottom of the containing vessel if it is not disturbed. Hence, the idiom refers to someone or something that is at ease, not disturbed, or worried. Sometimes this also has reference to a false assurance.
If a young man sows his wild oats, he has a period of his life when he does a lot of exciting things and has a lot of sexual relationships. for e.g. He'd spent his twenties sowing his wild oats but felt that it was time to settle down.
If something is a tough nut to crack, it is difficult to find the answer or solution. When used about a person, it means that it is difficult to get them to do or allow what you want.
'Hard nut to crack' is an alternative.