This is the 13740th most frequent German word.
engsten
"Engsten" is the inflected form of "eng" in German, meaning "tightest" or "closest" in English, depending on context.
In this sentence, 'engsten' is used as an adjective, derived from 'eng', meaning 'close' or 'tight', describing the friends as being very close and trusted.
Er vertraute nur seinen engsten Freunden seine Geheimnisse an.
He only trusted his closest friends with his secrets.
Here, 'engsten' is used as an adjective to denote 'tightest' or 'most constricted', describing the degree of traffic congestion.
Die Straße war für den engsten Verkehr gesperrt.
The street was closed for the tightest traffic.
In this usage, 'engsten' serves as an adjective to describe the circles as being the most exclusive or intimate groups.