cardinalyoudaoicibaDictYouDict[cardinal 词源字典]
cardinal: [12] The ultimate source of cardinal is Latin cardō ‘hinge’, and its underlying idea is that something of particular, or ‘cardinal’, importance is like the hinge on which all else depends. English first acquired it as a noun, direct from ecclesiastical Latin cardinālis (originally an adjective derived from cardō), which in the early church denoted simply a clergyman attached to a church, as a door is attached by hinges; it only gradually rose in dignity to refer to princes of the Roman Catholic church. The adjective reached English in the 13th century, via Old French cardinal or Latin cardinālis.
[cardinal etymology, cardinal origin, 英语词源]
cardinal (n.)youdaoicibaDictYouDict
early 12c., "one of the ecclesiastical princes who constitute the sacred college" (short for cardinalis ecclesiae Romanae or episcopus cardinalis), from Latin cardinalis "principal, chief, essential" (see cardinal (adj.)).

Ecclesiastical use began for the presbyters of the chief (cardinal) churches of Rome. The North American songbird (Cardinalis virginianus) is attested from 1670s, so named for its resemblance to the cardinals in their red robes.
cardinal (adj.)youdaoicibaDictYouDict
"chief, pivotal," early 14c., from Latin cardinalis "principal, chief, essential," from cardo (genitive cardinis) "that on which something turns or depends; pole of the sky," originally "door hinge," which is of unknown origin. Related: Cardinally.

The cardinal points (1540s) are north, south, east, west. The cardinal sins (c. 1600) are too well known to require rehearsal. The cardinal virtues (c. 1300) were divided into natural (justice prudence, temperance, fortitude) and theological (faith, hope, charity). The natural ones were the original classical ones, which were amended by Christians. But typically in Middle English only the first four were counted as the cardinal virtues:
Of þe uour uirtues cardinales spekeþ moche þe yealde philosofes. ["Ayenbite of Inwyt," c. 1340]
By analogy of this, and cardinal points, cardinal winds, cardinal signs (four zodiacal signs marking the equinoxes and the solstices), the adjective in Middle English acquired an association with the number four.