1580s, "keenness of intellectual perception, insight, acuteness of judgment;" see discern + -ment. From 1680s as "act of perceiving by the intellect."
Penetration, or insight, goes to the heart of a subject, reads the inmost character, etc. Discrimination marks the differences in what it finds. Discernment combines both these ideas. [Century Dictionary]
a man of discernment
discalceate
discard
discern
discernible
discerning
discernment
discharge
dischargeable
disciple
discipleship
disciplinable