Learn German: Meines Erachtens hat er allen Ernstes die Abfahrt verpasst. - In my opinion, he missed the departure in all seriousness.

This is an adverbial genitive, which is used in fixed phrases to express a state, time, or manner. Das Erachten is a neuter noun meaning opinion or judgment. Meines Erachtens literally translates to 'of my opinion', but in everyday German it functions as a standalone adverbial phrase meaning 'in my opinion'.

Is allen Ernstes also an adverbial genitive?

Yes! Just like Meines Erachtens, allen Ernstes (in all seriousness) is a fixed adverbial genitive phrase expressing manner. Der Ernst is a masculine noun, and the phrase functions as an adverb describing the surprising nature of how he missed the departure.

Why does the adjective in allen Ernstes end in -en instead of -es?

This is a specific rule for strong adjective endings in the genitive case. For masculine and neuter nouns that take an -s or -es ending in the genitive (like Ernstes), the preceding adjective takes an -en ending rather than -es. This prevents a repetitive, heavy-sounding 'es-es' combination.

Since Meines Erachtens and allen Ernstes are both adverbial phrases, why are they placed where they are in this sentence?