1. Lacking light or brightness and producing a dull, dark, or melancholy atmosphere.
2. Having a color or tone that is dark, dull, or suitable for a serious mood or occasion.
3. Marked by or conveying strict seriousness combined with sadness or a troubled state of mind.

Via French, "gloomy", ultimately from late Latin subumbrare, “to shadow" from sub, "under" plus umbra, "shade"; the source of English umbrage.