missive
/ˈmɪsɪv/noun
1- often humorous a letter, especially a long or official one〈常幽默〉（尤指冗长的）信件; 公函, 公文：
yet another missive from the Foreign Office.
来自外交部的另一封长函。
1.1
- Scots Law a document in the form of a letter interchanged by the parties to a contract【苏格兰律】合同公函。
词源
late Middle English （as an adjective, originally in the phrase LETTER MISSIVE）： from medieval Latin missivus, from Latin mittere 'send'. The current sense dates from the early 16th cent.