[ohm-BRR] (n. f.) In the classic 1850 book The Scarlet Letter, there's a vivid description of the adulteress Hester Prynne during a meeting with the minister, Arthur Dimmesdale, who aids her:
Throwing eyes anxiously in the direction of the voice, he indistinctly beheld a form under the trees clad in garments so somber and so little relieved from the gray twilight into which the clouded sky and the heavy foliage had darkened the noontide that he knew not whether it were a woman or a shadow.
If it sounds as though her somber garments are made of shadows, it's not by chance. Somber comes from the French word sombre, meaning obscure or melancholy. But the s- is very telling. It's a shortening of the Latin prefix sub which means under (think of the subcontractor who works on your home). That means that sombre is really sub-ombre and ombre is French for shadow which makes a lot of sense if you think of being somber as being under (emotional) shadows.
BONUS: What is it about that weird American-English tug-of-war over certain word spellings? Why is it theatre in the U.K. and theater in the U.S.? Well, it's just that the English, in an effort to remain true to roots, tend to stick with the French spelling of those words. Americans, meanwhile, try to spell things more closely to the way they sound phonetically. That's why somber is spelled the way it and not -re at the end, because it is pronounced like that. Like they say: KISS, keep it simple, stupid.