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Sex or masterbation before a nap.

Used as guise to get away from kids or family to have sex and possibly nap.
Mom, "Where are your grand parents?"

Child, " They said they where headed upstairs for a dirty nap."

"On the weekend, the old lady watches me like a hawk. A dirty nap is the only way I can get away to rub one out."
by DirtyDUSMC January 16, 2012