noun, な adjective
There are not any shakes or waves or anything fun like that. It is just pure dullness.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
My CD was suffering from a slump in sales for a little while, but since being introduced on the radio, sales are rising again this month.