usually pronounced "are ceez", the term is the plural of the letters of the acronym for research chemicals, see also rcs
as a slang term this would refer to the classes of substances not yet scheduled by the DEA as dangerous drugs, usually and particularly but not limited to, the large groups of Tryptamine and Phenethylamine compounds known for their psychoactivity but that have somehow evaded explicit mention in the DEA schedule.
This list would include AMT alpha, 5-meo-DMT deemers, 5-meo-DIPT foxy, and other tryptamine compounds as well as phenthylamine compounds such as 2C-E, 2C-I, 2C-T7 and others.
It is perhaps worth noting that at one time in the past, drugs such as LSD-25, mescaline and MDMA had the status of "research chemical" for varying lengths of time before being used by the population with such frequency as to arouse the attention of the authorities.
q: got any more of those rc's from last month?
a: naah man the government started scheduling whole classes of rcs about three weeks ago after some toad dipped his finger in the bag and died of an OD. now the chemists have started exploring the dreaded benzene ring in search of new variations.