Jai is one of the most used words in Thai. This one word means mind or heart but combined with other adjectives, verbs or nouns, it usually means state of mind. The credit of this post is not mine, but inspired by my great teacher.
Listed here are only few of the many words in Thai with Jai in it, and their meaning. The list is by far not complete!
Jai Rai - ใจร้าย bad heart - being mean
Jai Dee - ใจดี good heart - kind heart
Kao Jai - เข้าใจ into the heart - understand
Sabai Jai - สบายใจ carefree heart - being happy, worry-free
Sia Jai - เสียใจ lose a heart - being sad
Dee Jai - ดีใจ (good + heart) happy
Nam Jai - น้ำใจ water from the heart - kindness
Gin Jai - กินใจ eating the heart - emotionally touching
Jai Petch - ใจเพชร diamond heart - being determined, not giving up easily... a heart of stone?
Jai Khang - ใจแข็ง hard heart - being resistant, tough
Jai On - ใจอ่อน soft heart - being soft, giving in
Jai Mai Dee - ใจไม่ดี a heart that's not so good - feeling worried (e.g., I haven't heard from my son, who is a soldier, for few months now. Jai mai dee)
Jai Sun - ใจสั่น shaking heart - being scared, nervous, worried
Jai Ron - ใจร้อน hot heart - being impatient
Jai Yen - ใจเย็น cool heart - being patient, cool
Hai Jai - หายใจ (cure/lose + heart) - breathing
Jai Hai - ใจหาย losing heart - shocked, feeling frightening
Jai Pai Yuu Tee Ta Toom - ใจไปอยู่ที่ตาตุ่ม heart at the ankle - really frightened
Tok Jai - ตกใจ (fall + heart) - being surprised/startled (from loud sound), alarmed
Jai Sia - ใจเสีย a heart that's out of order - losing courage
On Jai - อ่อนใจ (fatigue/soft + heart) - being weary
Jai Dee Soo Sua - ใจดีสู้เสือ the good heart to fight a tiger - the state of mind when you're trying to overcome or hide your fear and do what has to be done despite that fear
Kreng Jai - เกรงใจ (in awe of + heart) - being considerate of the other person (that you will trouble him/her)
Klub Jai - กลับใจ (flip/turn + heart) - a change of heart, usually from bad to good
Plien Jai - เปลี่ยนใจ change mind - change mind
Tam Jai - ทำใจ (make+mind) - being settled with, usually negative, facts/news (e.g., Tum Jai with someone's behaviour or Tum Jai that you won't get your wallet back when it failed out of your pocket in the street)
Tun Jai - ทันใจ (reaching in time + heart) - usually used with speed when something is as fast as you want it to be
Dai Yaang Jai - ได้อย่างใจ (get + like + heart) - used when someone/something behave in the way you want him/her/it to be
Aow Tae Jai (Tua Eng) - เอาแต่ใจตัวเอง - serving one's heart only - used to describe someone who only do things to please him/herself and not considering anyone else's wishes or needs. This word doesn't mean selfish though.