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: