n. The side-effect that destroyed touch-screens as a mainstream input technology despite a promising start in the early 1980s. It seems the designers of all those spiffy touch-menu systems failed to notice that humans aren't designed to hold their arms in front of their faces making small motions. After more than a very few selections, the arm begins to feel sore, cramped, and oversized; hence 'gorilla arm'. This is now considered a classic cautionary tale to human-factors designers;
"Remember the gorilla arm!" is shorthand for "How is this going to fly in *real* use?".