brother-in-law (husband's brother)
Refers specifically to one's husband's brother. The term is somewhat dated and may carry a traditional or in-law family nuance; in modern casual speech, people often use 義理の兄 or 義理の弟 instead.
小舅にあたる人が、正月に訪ねてきた。
My husband's brother came to visit during the New Year holidays.
Compound of 小 (ko, 'small' or 'younger') and 舅 (shūto, 'father-in-law'), originally referring to a younger male relative by marriage. The exact historical derivation is uncertain, but the term has been used in traditional family contexts to denote a husband's brother.