catechesis (n.)
from Greek katekhesis "instruction by word of mouth," from katekhein "to instruct orally," originally "to resound" (with sense evolution via "to sound (something) in someone's ear; to teach by word of mouth." From kata- "down" (in this case, "thoroughly") + ekhein "to sound, ring," from ekhe "sound," from PIE *(s)wagh- "to resound" (see echo (n.)). Related: Catachectic; catachectical.