This is the 7065th most frequent Turkish word.
di
"Di" is a suffix in Turkish used to form the simple past tense of verbs.
'di' here is part of the past tense suffix '-di' attached to 'ilginç'. It denotes the completed state: 'was very interesting' in past tense.
Kitabı dün okudum; çok ilginçti.
I read the book yesterday; it was very interesting.
This usage of 'di' indicates a past tense description of a state: 'was uncomfortable'.
Görünüşe göre rahatsızdı.
Apparently, he/she/it was uncomfortable.
'di' is part of the passive past tense form '-ıldı', describing an action that happened in the past: 'was discussed'.