"sorrowful," early 15c., from French triste "sad, sadness" (10c.), from Latin tristis "sad, mournful, sorrowful, gloomy." Re-borrowed late 18c. (as "dull, uninteresting") as a French word in English and often spelled triste.
trisexual
triskaidekaphobia
triskelion
trismus
trisomy
trist
Tristram
trite
triticale
tritium
Triton