Translation

詳しく述べる / 長々と論じる / 詳説する

Meaning

Expatiate means to speak or write at length about a subject, often with many details. It is formal and usually appears with on or upon.

Examples

• 教授はその理論について長々と詳しく説明しました。
  The professor expatiated on the theory.
• 彼女は旅の楽しさについて詳しく語りました。
  She expatiated upon the pleasures of travel.