From Wiktionary, the free dictionary
Inflected stem of 太 (puto₁-).
太し (puto₁si) (kana ふとし)
- stable (of shape or mind)
c. 759, Man’yōshū, book 2, poem 190:真木柱太心者有之香杼此吾心鎮目金津毛- MAKI₂BASIRA PUTO₁KI₁ KO₂KO₂RO₂ PA ARIsikado₂ KO₂ NO₂ WA GA KO₂KO₂RO₂ SIDUme₂kanetu mo
- Though my heart is strong like tree pillars, I fail to be heartbroken.