Fossilised expression 'guten Mutes' (in good spirits)

guten Mutes means in good spirits / of good courage and is a fossilised genitive used adverbially or predicatively (often with sein). It conveys confidence and optimism. The genitive form is frozen and invariable.

• Guten Mutes begann er die Reise. - In good spirits he began the journey.
• Sie blieben guten Mutes. - They remained of good cheer.
• Trotz aller Rückschläge war er guten Mutes. - Despite all setbacks he was optimistic.

Common mistakes: Do not inflect or modernise it (mit gutem Mut is a normal phrase but not this idiom; gutes Mutes is wrong). It is a frozen relic and must be learned whole. Note its near-twin guter Dinge with the same cheerful sense but a different fixed form. The register is formal/literary.

'guten Mutes' (in good spirits) is a fixed genitive adverbial.

Fixed genitive adverbial 'in good spirits'; the form is frozen.

'guten Mutes' ist ein erstarrter Genitiv (adverbial).