Adieu/əˈduː/ Definition 1 of 2Pro Tip 1/3Parting Sentiment Used when expecting not to see someone again for a long time or ever. As she boarded the train, she called out 'adieu' to her family on the platform. Pro Tip 2/3Formal Tone 'Adieu' has a formal, poetic, or old-fashioned tone in English. They bade adieu with a formal handshake. Pro Tip 3/3Predominantly French While it's used in English, it's originally French and can sound sophisticated. He whispered 'adieu' with a perfect French accent.