mid-14c., "traveling, roving," from Anglo-French erraunt, from two Old French words that were confused even before they reached English: 1. Old French errant, present participle of errer "to travel or wander," from Late Latin iterare, from Latin iter "journey, way," from root of ire "to go" (from PIE root *ei- "to go"); 2. Old French errant, past participle of errer (see err). The senses fused in English 14c., but much of the sense of the latter since has gone with arrant.
-ancy
word-forming element denoting quality or state, from Latin -antia, forming abstract nouns on past-participle adjectives in -antem, appearing in English mostly in words borrowed directly from Latin (those passing through French usually have -ance or -ence; see -ance). But English also has kept many pairs of words in -ance and -ancy (radiance/radiancy, etc.). Though typically one of the two forms has been more common, both were kept "as metrically useful or rhetorically effective" [Fowler, 1926].