"Gird your loins and prepare for battle!" Okay, no one says "gird your loins" anymore (which basically means "tighten your pants"), but gird is still used as a verb to mean "get ready for a dangerous situation."
To gird is to prepare for a military attack, but more loosely it refers to readying oneself for any kind of confrontation. When you gird for something, you are preparing for the worst-case scenario. Gird can also mean "fasten something tightly with a belt or a band" (as in "gird your loins"), or it can mean "to surround or encircle." A field that is girded by trees is surrounded and encircled by trees.