IPA: /ˈtrɪstfəl/
KK: /trɪstfəl/
Feeling sad or unhappy; having a gloomy or sorrowful expression.
She had a tristful look on her face after hearing the bad news.
Comparative: more tristful
Superlative: most tristful
Tristful is formed from "trist" (from Latin "tristis", meaning sad or sorrowful) and "-ful" (meaning full of). The word "tristful" means full of sadness or sorrow.
Think of 'trist' meaning sad, and remember that 'tristful' describes someone who is full of sadness.