出典:Wiktionary
The noun is possibly derived from dull or Middle English dold (past participle of dullen, dollen (“to make または become blunt または dull; to make または become dull-witted または stupid; to make または become inactive”),[1] from dul, dol, dolle (“not sharp, blunt, dull; not quick-witted, stupid; lethargic, sluggish”);[2] see further at dull), modelled after tantrum.[3]
The adjective is probably derived from the noun.
doldrum (comparative more doldrum, superlative most doldrum)