Definition from Wiktionary, the free dictionary
ـٍ • (-in)
- kasra tanwīn (كَسْرَة تَنْوِين) or kasratān (كَسْرَتَان), a nunation (tanwīn (تَنْوِين)) of the vowel i (ـِ - kasra), indicating the genitive case in Classical Arabic. In a non-pausal formal situation it's pronounced as "-in". In dialects, relaxed (less formal) Arabic or in pausa, the pronunciation is either shorted to "-i" or ignored altogether, even when written.