The phrase "greased pig" refers to a game or activity where a pig is coated in grease to make it slippery. It is symbolic of a challenging or difficult task that is hard to catch or control. Full definition
We know that AVB likes a young squad of versatile players with lots of energy, able to play a high line, dominate possession and move like a greased pig when on the break. (whoframedruelfox.com)
With other sunscreens, I was either spending many more minutes than my toddler could tolerate trying to mash the thick goop around her face for full coverage, or I was trying to wrestle a greased pig into her shoes without shellacking my entire couch with sunscreen. (pregnancymagazine.com)