ا ث مʾ-th-m
(س) اَثِمَ يَاْثَمُ اِثْمًاathima yāthamu ithman: to sin, to fail to render what is due.
اِثْمٌithm (noun, ism dhāt): sin. ﴿فَلَآ اِثْمَ عَلَيْهِ﴾ (Al-Baqarah 2:173)
اٰثِمٌathim (active participle (ism fāʿil)): sinner. ﴿فَاِنَّهُۥٓ اٰثِمٌ قَلْبُهُۥ﴾ (Al-Baqarah 2:283) — here hu is the pronoun of the matter (ḍamīr al-shaʾn), that is: the fact is that his heart is sinful.