Literally: appropriate – mind / idea – instant – strange / delicate
Alternately: Clever; quick-thinking. Able to handle a situation (appropriately!) as soon as it arises.
Notes: This configuration is based on the Buddhist phrase 当位即妙. In this case, 位 refers to a (sometimes metaphorical) “position” rather than a mental state as 意 does, and the overall meaning is something more like “achieving enlightenment despite being a layperson.” From this came the meaning “quickly grasping the truth of a situation,” and a generalization to quick thinking or wit.
Compare near-synonym 臨機応変.
Replacing 意 with homophone 為 (“make,” “do,” “serve as” etc.) is considered an error.