The Rice University Neologisms Database

To consume cough syrup for its hallucinogenic effects Syrup, in the compound, has undergone a narrowing and, in the recent past, has undergone a less dramatic widening. First in narrowed from the class of all syrup to a specific brand of syrup, Robitussin, and, further, only that type that produces hallucinogenic effects. Soon, the word widened again to refer to any syrup offering hallucinogenic effects comparable to Robitussin. This compound was popularized in a new rap trend that began in Houston, and it is possible that the word ‘sippin’ was selected for its alliterative affects; the rappers typically do not sip the syrup: they chug it. It would be hard for the typical American to determine exactly what this compound referred to; but those in the know can easily see the connections.