also: ぼうげんたしゃ
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.
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.'
失言 is a noun meaning 'slip of the tongue' or 'gaffe,' while 妄言多謝 is a formal expression of apology for one's own reckless remarks.
謝罪 is a general apology, whereas 妄言多謝 is a set phrase specifically apologizing for thoughtless words in a humble, literary manner.
Standard kanji form for this yojijukugo.
Alternate reading; もうげんたしゃ is more common.
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.