Jiùshì shūxìn zhōng, " xián dì " shì duì wǎnbèi de jìngchēng.
In an old-style letter, 贤棣 is a polite way to address a younger man.
Tips
history
Originally a tree name, 常棣, a shrub the Book of Songs used as an image of brotherly closeness. From that poem the character also came to stand in for 弟 ('younger brother'), surviving in formal letter-writing terms like 贤棣.
The tree radical 木 on the left marks the original meaning: a kind of flowering shrub. Even when borrowed for 'younger brother', the plant root stays visible.