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A person who on the outside appears normal and acts normal. But in secret is a class A (Heroin / Crack / Meth etc) drug addict. The phrase comes from the R. Whites Lemonade television advertisment song of the 1970s / 80s as seen on British Television. In the advert a man is seen creeping downstairs at night in his pyjamas to drink lemonade from the fridge. Surprisingly there are quite a lot of Secret Lemonade Drinkers who are pillars of the community and occupy respectable professions - Doctors, Solicitors, Judges, Police Inspectors etc.
Yeah, did you hear about whats-his-name, apparently he is a "Secret Lemonade Drinker"! You wouldnt believe it of a vicar would you?
by Johnny Parrish August 19, 2011
A homosexual, who has yet to fully exit the closet, but every now and then has a little bit of a mince.
Rod - I swear Andrew keeps checking my cock out in the toilet!

Mick - funny I thought the same thing! I think he's a bit of a secret lemonade drinker.
by Wellhank December 01, 2012