WordNet sense 1 (a long-term economic state characterized by unemployment and low prices and low levels of trade and investment):

depression, economic crisis

WordNet sense 2 (a noticeable deterioration in performance or quality):

drop-off, falloff, slack, falling off

WordNet sense 3 (assume a drooping posture or carriage):

slouch

WordNet sense 4 (fall or sink heavily):

sink, slide down

WordNet sense 5 (go down in value):

correct, decline

WordNet sense 6 (fall heavily or suddenly; decline markedly):

fall off