Things that make you sad or depressed, or medications that slow down the nervous system; plural of downer.
From 'down' (low mood or position) + '-er' (one who causes). In the 1960s, 'downer' became slang for depressants like barbiturates. The figurative meaning (a disappointing event) developed from the drug slang.
The drug slang meanings and emotional meanings of 'downer' actually developed in tandem in the 1960s counterculture — when people took 'downers' (drugs), they got into a 'downer' (bad mood)! The word captures both the cause and effect.
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