Must've thought I was sleazy
Or though I was a mark cause I used to hang with Eazy

from Dr. Dre (Ft. Snoop Dogg) – Fuck Wit Dre Day Lyrics on Rap Genius

Meaning

Dre is lamenting about how his reputation may have been affected by being associated with Eazy-E.

Also, a “mark” is a sucker.

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