anything that can be nasty or deragatory. It's a sentence enhancer, you can replace it for almost anything. Usually is used for talking about farting or replacing the "S" word.
"Sorry guys i used all of the
toilet paper because i had to take a major skeff."
"Dude, I have been skeffing all day, maybe i should go
take a crap."
"My dog skeffed
all over the floor because i forgot to take him outside."