Green’s Dictionary of Slang

a depressing experience, a state of depression; often as the downs, a fit of depression; the antonym of up n.¹ (1)

1856		‘Cuthbert Bede’ Tales of College Life 101: He has been ‘all in the downs,’ lately.
1904		B.L. Farjeon Amblers 302: At present [...] the ‘downs’ were in the ascendant.
1938		F.D. Sharpe Sharpe of the Flying Squad 158: They regard it as a business, taking arrests as a little ‘down,’ and a good haul as a nice ‘up’.
1944		J. Cary Horse’s Mouth (1948) 256: But he’s always so up and down. Oh dear, I do hate his downs.
1969		Current Sl. IV:1.
1978		R. Price Ladies’ Man (1985) 120: A big down. I didn’t need this.
1983		J. Byrell (con. 1959) Up the Cross 34: They’re putting a down on the whole place.
1991		L. Bing Do or Die (1992) 34: ’Cause then you wouldn’t know what it feels like to have downs, to be at the bottom.
1999		Indep. 28 June 6: Prosac is supposed to counter-balance the depression and ‘down’ people feel after taking stimulants.