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It stands for "Hit Me Up If You're Down".

It's meaning is similar to that of HMU.

You would send this in a text or message to somebody you are interested in hooking up with and if they respond, proceed to assume they are "down" or "DTF".

It can also be used in group text or social media posts to get feedback from several people.
Some friends and I are going to the lake, whoever wants to come HMUIYD
by UrbanHerb September 24, 2015
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