solo B2

/ˈsoʊ.loʊ/ Alone, without anyone else.

sullen C2+

/ˈsʌlən/ badtempered and sulky; gloomy in mood or appearance

solitaire C2+

/ˈsɑləˌtɛr/ a single-player card game, or a single gem set alone

solitarily C2+

/ˈsɑləˌtɛrɪli/ in a solitary manner; alone and without company