contraction of "Semi-permanent nails"
Came from a mis-hearing of "I really need some more semi-permanent nails" it was thought that she'd said: "I really need some more sex"
also "doing your nails" is masturbation.
equally: "Doing his nails" would be a handjob or whatever.
"I need to get some nails"
"I'm sick of these boring nails"
"Go away! i'm doing my nails!"