'downer' originates from English, specifically the word 'down' combined with the agentive suffix '-er', used to form a noun meaning 'something that causes a state of being down'.

'downer' emerged in early 20th-century American English slang, developing from the verb 'to down' (originally 'to knock down, defeat') and later applied to drugs or things that 'bring one down'.

Initially, it meant 'something that knocks down or defeats', but over time it evolved into current senses: a depressant drug and something that causes sadness or disappointment.