Synonyms for Slump:

n.

diminish, crater, dwindle, shrink. trough, fallow, lull, failure, the slow lane, off-day, bad hair day, eclipse, wilderness. decline (noun)
descent, depreciation.
decline, failure (noun)
rut, collapse, descent, slip, drop, slide, fall, trough, depression, depreciation.
decrease (noun)
drop.
deterioration (noun)
descent.
slump (noun)
slack, depression, falling off, slouch, falloff, slide down, sink, drop off, fall off, economic crisis.

v.

decline, sink (verb)
slouch, decay, loll, tumble, plummet, topple, plunge, go downhill, fall off, deteriorate.
descend (verb)
come down, drop, tumble, lower, topple, fall, plunge, settle, plummet, descend, fall down.
deteriorate (verb)
regress, moulder, fade, ebb, retrogress, wane, fall apart, decay, worsen, lapse, sink, go downhill, revert, atrophy, putrefy, collapse, deteriorate, slip, get worse, degenerate, dilapidate, downgrade, decline, relapse, slide, tergiversate, wilt, wither.
motion (verb)
decline.

Other synonyms:

off-day, dwindle, cutback, crater, shrinkage, diminish, shrink, reduction. fallow, the slow lane, black out, faint, descent, loll. pass out, wilderness, eclipse, nose dive, lull, skid. dive, trough, failure. knock out. fit. cut. decrease
drop off.
descend
plummet.
drop-off
drop off.

Usage examples for slump

  1. Then, for a moment, my own spirit went slump with the realization of what lay before us, and a great weakness overcame me. – The Best Short Stories of 1921 and the Yearbook of the American Short Story by Various
  2. And the women turned away to their own- to their capital, which in the slump of Fate had suffered no loss. – The Judgment House by Gilbert Parker