perspicacious \pehr-spi-KEY-shehs\ (adjective) - Displaying insightful mental capacities and/or shrewd discernment.
"Deanne was very perspicacious to remain aloof from Jefferson when he started asking her to trust him."
From Latin perspicere "to look through" from per "through" + spicere "see, look." Akin to perspective. The Proto-Indo-European root underlying "spicere," *spek-, also gave us "spy." It metathesized (the consonants switched places) to skop- in Greek, whence all the Greek words on -scope borrowed by English: "telescope," "microscope," "periscope." The noun is "perspicacity" [per-spi-'ka-si-ti]. A shrewd person has a practical kind of intelligence and a sagacious one has knowledge usually accumulated over time. "Perspicacious" implies an ability to perceive hidden truths and to understand what puzzles others.