The act of ruining or soiling
something that it is preferred would be kept in nice condition. Traditionally applies to brand new
merchandise, cars, glossy or glass screened
electronics, and other consumables.
"I opened the box for my brand new $300
universal remote to find that some asshole had already gunked it up with his
skunk fingers and probably returned it."
"Oh shit, is that the new iPhone X*?? Can I see it???"
"No, you'll gunk it up with your skunk fingers!"
*Whatever
the latest COOOLEST one is.