The Collaborative International Dictionary of English v.0.48:
downer \downer\ n. a drug that reduces excitability and calms a person. [Colloq.] Syn: sedative, depressant. [WordNet 1.5]WordNet (r) 3.0 (2006):
downer n 1: a drug that reduces excitability and calms a person [syn: sedative, sedative drug, depressant, downer]