Made famous by the late Ronnie James Dio of Rainbow, Black Sabbath, Heaven and Hell and Dio. Dio would often shout at the top of his lungs in parts of a song "Look out!". Best used in context with a loud raspy voice while flashing the horns. Most commonly heard when describing an impending situation or event loosely connected to Heavy Metal.
"Look out" is also a phrase used in tribute to Dio, often yelled by rockers before they down a shot, take a drink or on hearing the opening riff of a Dio song.