A phrase used to describe the favored method of killing an enemy unit
that is powerful enough to do you damage. In the case of Main Battle Tank
s, this is a killing shot which comes from the rear where a tank's armor
is thinnest and where (hopefully) the target tank either hasn't seen the attacker or doesn't have time to rotate its turret
and train in
its gun before being hit.
This phrase is used extensively in David Weber's excellent and entertaining space opera the Honor Harrington series. In this case, it describes a tactic of starship combat. Weber's starships in this particular universe are, as a result of peculiarities of physics as applied to their drive and shielding systems, vulnerable in the front and rear of the vessel. The rear 'opening' of their shielding, however, must be larger if they are moving forward, which makes the shot easier.
As to the phrase itself, well, just recall that kilts are traditionally worn without underwear...