As in utsāha उत्साह - Resolve, Strength, or Will-Power
[from utsa; spring, fountain]
or matsara (मत्सर), jealousy
嗟字門，入諸法嗟字不可得故
Kumarajiva
The syllable TSA is a gateway because the intoxication / jealousy (matsara) of all dharmas cannot be attained.
入蹉字門，悟一切法勇健性不可得故
The syllable TSA is a gateway because the strength (utsāha) of all dharmas cannot be attained.
Xuanzang
TSA because will-power (utsāha) cannot be apprehended.
Conze
The syllable SA [means] all mental phenomena are openings because of the unattainability of the strength (utsāha),
Jayarava