"one not belonging to a regular body" of any sort, "one not subject to or not conforming with established regulations," 1610s, from irregular (adj.). Main modern sense of "a soldier not of the regular army" is from 1747.
Doubtless, the life of an Irregular is hard; but the interests of the Greater Number require that it shall be hard. If a man with a triangular front and a polygonal back were allowed to exist and to propagate a still more Irregular posterity, what would become of the arts of life? Are the houses and doors and churches in Flatland to be altered in order to accommodate such monsters? [Edwin Abbot, "Flatland: A Romance of Many Dimensions," 1885]
sacred banner of St. Denis, mid-15c., oriflamble, from Old French orie flambe, from Latin aurea flamma "golden flame." The ancient battle standard of the kings of France, it was supposed to have been of red or orange-red silk, with two or three points, and was given to the kings by the abbot of St. Denis on setting out to war. Cotgrave says it was "borne at first onely in warres made against Infidells; but afterwards vsed in all other warres; and at length vtterly lost in a battell against the Flemings." It is last mentioned in an abbey inventory of 1534.
Biblical title of honor, literally "father," used as an invocation of God, from Latin abba, from Greek abba, from Aramaic (Semitic) abba "the father, my father," emphatic state of abh "father." Also a title in the Syriac and Coptic churches.
It is used in the New Testament three times (Mark xiv. 36, Rom. viii. 15, Gal. iv. 6), in each instance accompanied by its translation, "Abba, Father," as an invocation of the Deity, expressing close filial relation. Either through its liturgical use in the Judeo-Christian church or through its employment by the Syriac monks, it has passed into general ecclesiastical language in the modified form of abbat or abbot .... [Century Dictionary]