adj. sul·len·er, sul·len·est
1. Showing a brooding ill humor or silent resentment; morose or sulky.
2. Gloomy or somber in tone, color, or portent: sullen, gray skies.
3. Sluggish; slow: the sullen current of a canal.
[Middle English solein, from Anglo-Norman solein, alone, from sol, single, from Latin sōlus, by oneself alone; see s(w)e- in the Appendix of Indo-European roots.]
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Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.