A person you would say "hi" and "bye" to casually, but little more. Generally someone desirable as an acquaintance and not anything more.
From "This American Life"
After she said "no" to a date, she said we could remain hi bye friends.
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Hi sup yo hi hi sup yo sup sup yo hi sup sup hi sup bye 😽 hi 🙋 ♀️ I got a call from you I love 💗 and orange I know you guys have fun to watch your
Hi sup sup hi hi bye hi sup yo sup hi
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It means that when you going around in a circle and you see somebody you say hi-bye
“Hi-bye!”