What does the phrase "to armor oneself with vows" mean? This seems to be a common trope-- samnaha. I honestly have no idea what they mean, it strikes me as a "it's raining cats and dogs" sort of idiom. Is there an explanation for what it means?

Examples:


  For they wear the armor of great vows, accumulate merits, deliver all
  beings from birth-and-death, visit Buddha-lands to perform the
  bodhisattva practices, make offerings to Buddhas


http://www.onmarkproductions.com/html/48-bosatsu-vows.shtml

And in Shingon, they visualize themselves putting on armor (or physically don?)

Thinking Through Shingon Ritutual
"Then one protects the body by donning armor..."
What does the phrase “to armor oneself with vows” mean?

I don't recall seeing that exact phrase in the texts that I've read but the analogy of the vows being used as an armor symbolizes the protection they provide to oneself and others by observing them. You can see this theme in the Self-protected Discourse (Atta-rakkhita Sutta) for example:


  At Savatthi. As he was sitting to one side, King Pasenadi Kosala said
  to the Blessed One: "Just now, lord, while I was alone in seclusion,
  this train of thought arose in my awareness: 'Who have themselves
  protected, and who leave themselves unprotected?' Then it occurred to
  me: 'Those who engage in bodily misconduct, verbal misconduct, &amp;
  mental misconduct leave themselves unprotected. Even though a squadron
  of elephant troops might protect them, a squadron of cavalry troops, a
  squadron of chariot troops, a squadron of infantry troops might
  protect them, still they leave themselves unprotected. Why is that?
  Because that's an external protection, not an internal one. Therefore
  they leave themselves unprotected. But those who engage in good bodily
  conduct, good verbal conduct, &amp; good mental conduct have themselves
  protected. Even though neither a squadron of elephant troops, a
  squadron of cavalry troops, a squadron of chariot troops, nor a
  squadron of infantry troops might protect them, still they have
  themselves protected. Why is that? Because that's an internal
  protection, not an external one. Therefore they have themselves
  protected.'"
  
  "That's the way it is, great king! That's the way it is! Those who
  engage in bodily misconduct, verbal misconduct, &amp; mental misconduct
  leave themselves unprotected. Even though a squadron of elephant
  troops might protect them, a squadron of cavalry troops, a squadron of
  chariot troops, a squadron of infantry troops might protect them,
  still they leave themselves unprotected. Why is that? Because that's
  an external protection, not an internal one. Therefore they leave
  themselves unprotected. But those who engage in good bodily conduct,
  good verbal conduct, &amp; good mental conduct have themselves protected.
  Even though neither a squadron of elephant troops, a squadron of
  cavalry troops, a squadron of chariot troops, nor a squadron of
  infantry troops might protect them, still they have themselves
  protected. Why is that? Because that's an internal protection, not an
  external one. Therefore they have themselves protected."


There are others discourses that touch on this. I particularly have one in mind that relates very closely to this theme. I'll reference when I find it.