| somnolent | sleepy; drowsy | |
| sophomoric | Pertaining to or characteristic of a sophomore; considered to be overconfident, conceited, and exhibiting a lack of maturity | |
| soporific | Tending to induce drowsiness or sleep | |
| sordid | involving ignoble actions and motives; arousing moral distaste and contempt; dirty or squalid | |
| soupcon | A very small quantity or trace of something | |
| sovereign | A supreme ruler, especially a monarch. | |
| sparing | Given to or marked by the restrained and efficient use of resources; careful not to waste or be extravagant | |
| sparse | Thinly dispersed or scattered | |
| specious | Seemingly plausible, but actually wrong | |
| spendthrift | a person who spends money in an extravagant, irresponsible way | |