Stand From Under Meaning

(idiomatic) to escape something falling or being thrown from above.

Example: "Stand from under," he shouted out, as he threw down a cocoa-nut, which very nearly hit Billy, who had not attended to his warning. — W.H.G. Kingston, "The Three Admirals", 1873
I see I was in a fix now. But it warn't no use to worry; there warn't nothing to do but just hold still, and try and be ready to stand from under when the lightning struck. — Mark Twain, "The Adventures of Huckleberry Finn", 1884