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Not a euphemism for male masturbation, but rather a popular exposition of time-travel paradox:
You enter a time machine, and set the date for, say, 50 years in the past, after your grandfather was born but before your father was born. You find Grandpa and kill him. Since there is no Grandpa, there can be no Dad, and no "you": you have altered history to make yourself an anomaly.
So what happens? There are several theories to cope with this paradox: