/r/Army has a thread about the worst unit motto, and the 507th's was brought up. They even make unit coins and crests with it. No one seems to know where this very unusual motto comes from, so I'm trying here.
I bet if you asked them you'd get an answer.
https://www.facebook.com/pages/507th-Engineer-Battalion/112662815411095
The U.S. Army Heritage Center usually keeps this sort of information, on distincitive unit insignia, mottos, etc. However, the website has been unresponsive for me today. The link is here, in case it comes back up.
Worst case, you could try emailing or calling one of the researchers there. If anyone can find out, it's them.
I reached out to a friend who was in the 507th Engineer Battalion who looked into it for me. His response:
From what I can find out. The motto dates back to the civil war however was not adopted until the 70s.
Our lineage dates back to the 1st Regiment of Michigan Engineers and Mechanics. During the siege of Atlanta this unit encountered confederate General Wheelers cavalry and were able to set up a defense and repel 7 waves of cavalry attacks. General Wheeler sent a correspondence requesting the surrender of their group. And they responded something to the effect of "as long as we have the means to fight, I cannot see it."