‘岳弟’ (yuè dì) is a formal, respectful term used to refer to one’s wife’s younger brother. The character ‘岳’ (yuè) literally means ‘mountain’ but in kinship terms specifically denotes the wife’s family—hence ‘岳父’ (yuèfù) for father-in-law and ‘岳母’ (yuèmǔ) for mother-in-law. The character ‘弟’ (dì) means ‘younger brother’. Together, ‘岳弟’ literally conveys ‘younger brother of the wife’s family’, distinguishing him from one’s own younger brother (弟弟, dìdi) or elder brother’s younger brother (堂弟, tángdì).

This term is commonly used in spoken and written Chinese when referring to the relationship formally—such as in introductions, wedding speeches, family letters, or official documents—and reflects traditional Chinese emphasis on precise kinship terminology. It is not used as a direct form of address (i.e., you wouldn’t call him ‘岳弟’ to his face); instead, people usually say ‘小舅子’ (xiǎo jiùzi) informally or simply use his given name with appropriate honorifics.