the.com/downer

means A downer is something — a drug, a comment, an event — that drags your mood down into disappointment or depression.

from Emerges in mid-20th-century slang, built straightforwardly from 'down' plus the agentive '-er' (the thing that does the downing). It surfaced first among drug users to name sedatives like barbiturates — the literal opposite of an 'upper' — then widened by the 1960s–70s to cover any person, event, or vibe that deflates a mood. The drug sense and the mood sense grew up together, each reinforcing the other.