late 12c., "sufferings of Christ on the Cross," from Old French passion "Christ's passion, physical suffering" (10c.), from Late Latin passionem (nominative passio) "suffering, enduring," from past participle stem of Latin pati "to endure, undergo, experience," a word of uncertain origin.
Sense extended to sufferings of martyrs, and suffering generally, by early 13c.; meaning "strong emotion, desire" is attested from late 14c., from Late Latin use of passio to render Greek pathos. Replaced Old English þolung (used in glosses to render Latin passio), literally "suffering," from þolian (v.) "to endure." Sense of "sexual love" first attested 1580s; that of "strong liking, enthusiasm, predilection" is from 1630s. The passion-flower so called from 1630s.
The name passionflower -- flos passionis -- arose from the supposed resemblance of the corona to the crown of thorns, and of the other parts of the flower to the nails, or wounds, while the five sepals and five petals were taken to symbolize the ten apostles -- Peter ... and Judas ... being left out of the reckoning. ["Encyclopaedia Britannica," 1885]