"involuntary muscular movements of hollow organs of the body," especially the alimentary canal, 1859, from Modern Latin peristalsis; see peristaltic.
mid-15c., "duration, continuance" (a sense now obsolete; probably an abbreviated form of endurance); sense of "imprisonment, restraint of the person, involuntary confinement" is from 1510s, from Old French durance "duration," from durer "to endure," from Latin durare "to harden," from durus "hard," from PIE *dru-ro-, suffixed variant form of root *deru- "be firm, solid, steadfast."
1550s, "stroke of a whip," from jerk (v.1). Sense of "sudden sharp pull or twist" is by 1570s. Meaning "involuntary spasmodic movement of limbs or features" recorded from 1805. As the name of a popular dance, it is attested from 1966.
c. 1500, "reflection of light, image produced by reflection," from a verb reflex meaning "refract, deflect" (late 14c.; compare reflect), from Late Latin reflexus "a bending back," noun use of past participle of reflectere "to bend back, bend backwards, turn away," from re- "back" (see re-) + flectere "to bend" (see flexible). Also as an adjective (1640s), "thrown or turned backward," also of thoughts or the mind. Meaning "involuntary nerve stimulation" is recorded by 1877, short for reflex action (1833) "simple, involuntary action of the nervous system."
"involuntary motion of the eyes," 1790, medical Latin, from Greek nystagmos "nodding, drowsiness," from nystazein "to nod, slumber, be sleepy," from PIE *sneud(h)- "to be sleepy." Beekes compares Baltic words such as Lithuanian snūsti "to slumber away." Related: Nystagmatic.