A "spitter" is a dose of drugs that is found out by taste not to be genuine and therefore spat out.
For instance an Ecstasy pill that is distinctly bitter or causes tongue numbing or a cold sensation is to all likeliness contaminated or not genuine at all. Similarly a blotter
which tastes bitter probably does not contain LSD
and given the specific preferences of many users of psychedelics
they are often just spat out with no second thought given.