mid-15c., occasionen, "to bring (something) about, be the cause of (something)," from occasion (n.), or else from Old French occasionner "to cause," from Medieval Latin occasionare, from Latin occasionem (see occasion (n.)). Related: Occasioned; occasioning.
1560s, "cause to go down," from down (adv.). Meaning "swallow hastily" is by 1860; football sense of "bring down (an opposing player) by tackling" is attested by 1887. Figurative sense of "defeat, get the better of" is by 1898. Related: Downed; downing.
late 14c., "act of bringing into existence," from Old French introduccion (14c.) and directly from Latin introductionem (nominative introductio) "a leading in," noun of action from past-participle stem of introducere "to lead in, bring in; introduce; found, establish; bring forward (as an assertion)," from intro- "inward, to the inside" (see intro-) + ducere "to lead" (from PIE root *deuk- "to lead").
Meanings "initial instruction in a subject" and "an introductory statement" are from mid-15c.; meaning "elementary treatise on some subject" is from 1520s. The sense of "formal presentation of one person to another" is from 1711.