noun
Refers to the background, circumstances, or story behind something, often with a nuance of complication or hidden details. Common in phrases like 曰く付き (with a story/background) or 曰く因縁 (complicated history).
この家には曰くがあるらしい。
This house seems to have a story behind it.
彼は曰く付
He bought an item with a history at a low price.
adverb
according to ...; ... says
Used to quote or report what someone said, often in formal or literary contexts. Functions like 'so-and-so says' or 'according to so-and-so'. Typically follows a name or source.
先生曰く、この問題は試験に出るそうだ。
According to the teacher, this problem will appear on the exam.
彼曰く、それは誤解だとのことです。
He says it's a misunderstanding.