Means you’re feeling properly miserable, low, or a bit depressed, like your whole vibe’s been binned. It can be a temporary slump after bad news or just one of those days where even tea tastes tragic. Not super edgy slang, more a classic everyday phrase that still gets used loads.
"He’s been down in the dumps since his missus dumped him, so we dragged him to the pub for a pint and a moan, but he just sulked in the corner."