A roof unicorn refers to a man with a shovel in his pants (giant penis).
Example 1:

We were in the locker room getting dressed and I saw Marquavius’s nine inch penis, and that was soft! He’s a total roof unicorn!

Example 2:

Guy: What’s she doing with a guy like that?! He’s so ugly!
Guy 2: I heard he’s a total roof unicorn.
by RoofUnicorn69 November 5, 2020
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