What it means
Calling someone an icon is high, affectionate praise, hailing them as a legend whose style, attitude or moment is unforgettable and worth celebrating. You absolute icon.
Usage examples
"She wore sunglasses indoors and owned it, total icon."
"You organised the whole trip single-handed, icon."
"You fixed your eyeliner on the train with a spoon and still looked unreal, icon."
"He turned up late with coffees for everyone and somehow saved the whole morning, what an icon."
Where it comes from
It comes from the regular word icon, originally a holy image or a figure people look up to. In modern English it widened into pop culture and everyday hype, so now you can call someone an icon when they feel instantly memorable, admired, and bigger than the room for a minute.