"Hang" or "hanging" as a shortened form of "hang out" or "hanging out" has been commonly used in AmE since at least the 1980s. My evidence is strictly anecdotal, but when I was growing up in the 1970s most folks still said "hang out": "What are your plans today?" "Not much, probably just going to hang out with my friends." But I heard "hang" creep into the vernacular in the early 1980s and suddenly folks (at that time, usually younger folks) would reply instead "Not much, probably just going to hang with my friends." Today, people of all ages use "hang" in that context.
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