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When a girl is giving head and her top lip goes under her teeth, and gives her the appearance of an old woman missing her dentures.
"Damn last night kelsey was giving me head and when I looked down she was grandma lippin me!"

"Ew thats gross man!"
by Osgoodslaughters June 18, 2009
When a person of either gender has unusually dry, sometimes ribbed lips, which somehow are beyond the use of chapstick. Grandma lips absolutely suck to be in contact with, and in the event of finding yourself in the situation of being with a girl/guy that has grandma lips, get the hell out immediately.
I was taking one for the team last night when I hooked up with that girl Erin, she had terrible grandma lips. My carmex was useless, and I was out in an instant.
by watup taz June 04, 2006