also: ぼうげんたしゃ
expression
please excuse my reckless remarks; kindly forgive my thoughtless words
A formal, self-deprecating yojijukugo used deferentially in letters or speeches to apologize for one's own careless or arrogant statements.
先日の発言につきましては、妄言多謝と存じます。
Regarding my remarks the other day, I must apologize for my reckless words.
手紙の末尾に「妄言多謝」と記した。
At the end of the letter, I wrote 'please excuse my thoughtless words.'
A yojijukugo composed of 妄言 (reckless talk) and 多謝 (many thanks/apologies). The exact historical origin is uncertain, but it is used as a set phrase in formal correspondence.