crusade n.

1814, "a cross," from Latin crux "cross," a word of uncertain origin. Sometimes said to be cognate with Irish cruach "heap, hill," Gaulish *krouka "summit," Old Norse hryggr "backbone," Old English hrycg "back." But de Vaan is suspicious:

The Celtic and Gm. forms are often reconstructed as *kr(e)u-k-, but we find vacillating vocalism within Gm.; also, the meanings 'backbone' and 'heap' are not necessarily connected. Even if the words in *kruk- from Latin and Italo-Celtic belong together, the root structure does not look PIE (and a root enlargement k is unknown), and might be interpreted as a non-IE substratum word borrowed into Italo-Celtic. But Latin may also just have borrowed the word from a contemporary language.

The figurative use for "a central difficulty" (1718) is older in English than the literal sense; perhaps it is from Latin crux interpretum "a point in a text that is impossible to interpret," the literal meaning of which is something like "crossroads of interpreters." But Century Dictionary ascribes it to "the cross as an instrument of torture; hence anything that puzzles or vexes in a high degree ...." Extended sense of "central point" is attested by 1888.

Old English cros "instrument of Christ's crucifixion; symbol of Christianity" (mid-10c.), probably from Old Norse or another Scandinavian source, picked up by the Norse from Old Irish cros, from Latin crux (accusative crucem, genitive crucis) "stake, cross" on which criminals were impaled or hanged (originally a tall, round pole); hence, figuratively, "torture, trouble, misery;" see crux. Also from Latin crux are Italian croce, French croix, Spanish and Portuguese cruz, Dutch kruis, German Kreuz. 

The modern word is the northern England form and has predominated. Middle English also had two other forms of the same word, arriving from the continent by different paths: cruche, crouche (c. 1200) was from Medieval Latin, with pronunciation as in Italian croce (compare Crouchmas "festival of the Invention of the Cross," late 14c.). Later, especially in southern England, the form crois, croice, from Old French, was the common one (compare croisade, the older form of crusade). The Old English word was rood.

By c. 1200 as "ornamental likeness of the cross, something resembling or in the form of a cross; sign of the cross made with the right hand or with fingers." From mid-14c. as "small cross with a human figure attached; a crucifix;" late 14c. as "outdoor structure or monument in the form of a cross." Also late 14c. as "a cross formed by two lines drawn or cut on a surface; two lines intersecting at right angles; the shape of a cross without regard to religious signification." From late 12c. as a surname.

From c. 1200 in English in the figurative sense "the burden of a Christian; any suffering voluntarily borne for Christ's sake; a trial or affliction; penance in Christ's name," from Matthew x.38, xvi.24, etc. Theological sense "crucifixion and death of Christ as a necessary part of his mission" is from late 14c.

As "a mixing of breeds in the production of animals" from 1760, hence broadly "a mixture of the characteristics of two different things" (1796). In pugilism, 1906, from the motion of the blow, crossing over the opponent's lead (1880s as a verb; cross-counter (n.) is from 1883). As "accidental contact of two wires belonging to different circuits," 1870.

person engaged in a crusade, 1743, from crusade (n.) + -er (1). Earlier was croisader, from French croisadeur. In Middle English a crusader might be called a pilgrim.