perspicacity (n.) Look up perspicacity at Dictionary.com
1540s, from Middle French perspicacité (15c.) and directly from Late Latin perspicacitas "sharp-sightedness, discernment," from Latin perspicax "sharp-sighted, having the power of seeing through," from perspicere "look through, look closely at" from per "through" (from PIE root *per- (1) "forward," hence "through") + specere "look at" (from PIE root *spek- "to observe").