Sullen is a word which has been derived from Middle English. The words from which it is derived are solein from English and solein from Anglo-Norman also. It is also derived from Latin words that are sol, and single. They all refer to one meaning that is alone and to be by oneself. It usually refers to showing a threatening ill humour. It is also considered to show a silent discomfort or bitterness with something or someone. The person may act serious or gloomy in his/her tone. For things it may be applicable as anything that seems dull, morose, fading colours etc. it may also refer to being sluggish. The relative terms are sullenly (adjective), and sullenness (noun).