Fossilised expression 'guter Dinge' (in good cheer)

guter Dinge sein means to be cheerful / optimistic / in good spirits and is a fossilised genitive idiom, typically used predicatively with sein or adverbially. The genitive (an old partitive of good things) is frozen and invariable.

• Guter Dinge gingen sie nach Hause. - In good cheer they went home.
• Sie waren guter Dinge. - They were optimistic.
• Trotz der Lage blieb er guter Dinge. - Despite the situation he stayed cheerful.

Common mistakes: The noun is plural genitive Dinge, not singular - never guten Dings. Do not inflect or add a preposition. It overlaps in meaning with guten Mutes, but each has its own fixed form, so do not blend them (guter Mutes / guten Dinge are wrong). The register is formal/literary.

'guter Dinge sein' (to be cheerful/optimistic) is a fixed genitive idiom.

Fixed genitive 'cheerful/optimistic' (guter Dinge sein); the form is frozen.

'guter Dinge sein' (zuversichtlich sein) ist eine erstarrte Genitivwendung.