襟 is a bound form (b.f.) that originally referred to the front part of a traditional Chinese garment that overlaps and is fastened. In modern Chinese, it appears almost exclusively in compound words. Its most common uses are in terms related to one's 'bosom' or 'mindset' (e.g., 胸襟 'breadth of mind') and in kinship terms for in-laws (e.g., 连襟 'husbands of sisters'). You won't use 襟 alone as a standalone word; it always attaches to another character.