Look out, behind you!
Someone is warning you of something coming at you from your blind side. I.e., the side of you that you can't see. Thanks to the design of the human eye, that actually covers most sides of you at any given moment. (In fact, there is a built-in blind spot in each of your eyeballs; see scotoma.) If you can ever see directly behind you, thank (or curse) your local chiropractor.
But C-Dawg, I hear you complain, that's two words, and this is a one-word node. Yes, in its nounal persona, "blind side" is two words (and could, but never actually does, refer to that 2009 movie with the fine Sandra Bullock, "The Blind Side"), but the more prevalent appearance is as a verb.
To blindside is to collide with a person, approaching from their blind side. While you could blindside John Doe, it's almost always Doe's fault: he blindsided you. And these days, there's a good chance that a motor vehicle or two were involved.
But then, the verb rarely refers to an actual geographical event. Rather, it's most likely used in the sense of a totally unexpected, even almost unexpectable, action on the part of a friend, a foe, or even the Universe as a whole. "I was totally blindsided by [that jerk getting the promotion][inheriting a million dollars from Great Aunt Sadie]."
One can also be blindsided by an idea springing from nowhere. While that's generally a bad thing, happier (or at least, neutral) thoughts can be described by a different spatial metaphor: to "come out of left field". (Why not right field? I dunno.) It can also "come out of the blue"; maybe from absentmindedly looking at the sky?
Side Quest 2024