What does 어언간 (於焉間) mean?

어언간 (於焉間) literally means at (於) some (焉) time (間). It is equivalent to the pure Korean expression 어느새에 (어느 사이에), which translates as in no time at all, before one knows it, or already. It implies surprise at the quick passage of time. The Korean definition for 어느새 is 어느 틈에 벌써, which literally translates as already at some interval.

• 어 (於) is the Chinese equivalent of the Korean 에, which translates as at.
• 언 (焉) is the Chinese equivalent of the Korean 어느, which besides translating as which can also translate as some.
• 간 (間) is the same character used in 시간 (時間), which is the Korean word for time. The character 間 (간) can translate as interval or time.

The adverb 어언간 (於焉間) can be reduced to 어언 (於焉) without any lose in meaning. In fact, today I came across the following sentence:

은나라가 성립된 것이 B.C. 1500년 경이니, 지금으로부터 어언 3500년 前.

 The Yin dynasty (은나라) was established at about B.C. 1500, so it has already been 3500 years.