[(mostly)
nuclear strategy]:
First strike against an opponent who
appears committed to an
attack against you.
If they were to attack you, you'd
retaliate. Only you'd stand the
risk of being too flattened to do so. Instead, you "retaliate" before they attack.
Hypocritical as it may sound, this term may actually have been in use...
Then again, it's more likely an urban legend.