Hi, Deadman and all.
Your search engine, believe it or not, is a very good thesaurus. At least DuckDuckGo and Google are. Try “slang term for [fill in the blank] if the thesaurus synonyms don’t work for you.
Beyond my suggesting something like the “happy trail” for between the navel and the bikini line, or ‘below the bikini line,’ etc if the area is there. If it’s actually someone’s lap, try ‘free parking’ or ‘the empty parking place.’
Cheers.