a.k.a Intertropical Convergence Zone (ICZ)
Early sailors named this belt of calm the doldrums because of the low spirits they found themselves in after days of no wind. To find oneself becalmed in this region could mean death in the era when wind was the only motive force available, and it was certainly a depressing experience to be isolated, in the middle of the ocean, in a hot, muggy climate.
The adjective form of the prepositional phrase "in the doldrums." It is used to express the definition of Doldrums (a dull, listless, depressed mood; low spirits) in an adjective form.