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50 Cent has new legal trouble after taunting a 19-year-old man who turned out to be autistic -- TMZ has learned his family is strongly considering a lawsuit.

Andrew Farrell was just doing his job as a janitor at the Cincinnati/Northern Kentucky airport when 50 Cent appeared to randomly single him out on video ... accusing him of being high at work. Fiddy posted the video on Twitter.

Ken Kramer, Andrew's stepdad, tells us Andrew suffers from autism and Asperger's syndrome. He also has a hearing condition which forces him to wear a hearing aid.

Ken says Andrew has never smoked or had a sip of alcohol in his life. He says Andrew has severe social anxiety, and that's why he ignored 50 -- he also doesn't know who the rapper is.

Ken says the family has lawyered up, and the only thing stopping them from suing immediately is concern about stiff attorney fees. He says they'd be willing to forget the whole thing, IF 50 Cent donates $1 million to Andrew's GoFundMe page, and makes a face-to-face apology.

They might want to research 50's finances.