discipline (n.)

Old English discipul (fem. discipula), "one who follows another for the purpose of learning," especially "the personal followers of Jesus Christ during his life, the twelve Apostles chosen or called by him to be his immediate associates," a Biblical borrowing from Latin discipulus "pupil, student, follower," which is of uncertain origin.

In OED and Watkins it is said to be from discere "to learn," from a reduplicated form of the PIE root *dek- "to take, accept." But according to Barnhart and Klein, it is from a lost compound *discipere "to grasp intellectually, analyze thoroughly," from dis- "apart" (see dis-) + capere "to take, take hold of," from PIE root *kap- "to grasp." De Vaan finds the ending -pulus "difficult to explain" in the former theory and the latter "semantically not compelling."

It was not common in Old English, where the usual word was leorningcniht, and in some cases þegn (see thane). The pre-Christian Latin sense of "scholar, pupil, student" is rare in English. Meaning "one who follows or is influenced by the doctrine or example of another" is from c. 1300.