The eminent ammbassador was but an ___(average)___ linguist;yet he insisted in speaking to foreign dignitaries in their own tounges.

why is it average?

Doesnt was BUT mean opposite?

http://www.americanpoems.com/poets/emilydickinson/10689

In that poem, the first line means
The moon was just a arc of light, ...

was but does not mean opposite. It means was simply

So in your sentence, the lofty ambassador was not really fluent in languages, but he insisted on...

http://dictionary.cambridge.org/define.asp?key=10516&dict=CALD

thanks!

The word "but" in this context is used to indicate contrast or contradiction. In this case, it suggests that although the eminent ambassador was highly regarded in their profession, their linguistic skills were merely average. So, the word "average" describes the level of proficiency the ambassador had in speaking different languages, indicating that it was not exceptional or outstanding, but rather ordinary.

Regarding your confusion about "but" indicating the opposite, it's important to note that "but" is often used to introduce a contrasting or opposing element. It can show a contrast between two things that seem contradictory. In this sentence, it contrasts the ambassador's highly regarded status with their average language skills, highlighting the contradiction between their reputation and their linguistic abilities.