noun
Often used in negative constructions like いわれはない (there is no reason) or いわれのない (unfounded, unwarranted).
彼に責められるいわれはない。
There is no reason for him to blame me.
それはいわれのない非難だ。
That is an unfounded accusation.
noun
Refers to the story or circumstances behind something, often a name, tradition, or object. Common in phrases like いわれのある (having a history/story).
この神社には古いいわれがある。
This shrine has an old history.
その地名のいわれを調べた。
I looked into the origin of that place name.
理由 is the general word for 'reason' and is more common in everyday speech. いわれ is more literary and often carries a nuance of historical or traditional justification.
由来 also means 'origin' or 'history', but is more formal and often used in written explanations. いわれ is slightly more colloquial and can imply a story or legend.
Derived from the classical Japanese verb 言う (iu, 'to say') in its passive form 言われる (iwareru), meaning 'it is said'. The noun いわれ originally meant 'what is said about something', evolving to mean 'reason' or 'history'.