"life of idle begging," 1706, from vagrant + abstract noun suffix -cy. Earlier in a figurative sense, "mental wandering" (1640s). By late 18c. used in law as a catch-all for miscellaneous petty offenses against public order.
mid-15c., "person who lacks regular employment, one without fixed abode, a tramp," probably from Anglo-French vageraunt, also wacrant, walcrant, which is said in many sources to be a noun use of the past participle of Old French walcrer "to wander," from Frankish (Germanic) *walken, from the same source as Old Norse valka "wander" and English walk (v.).
Under this theory the word was influenced by Old French vagant, vagaunt "wandering," from Latin vagantem (nominative vagans), past participle of vagari "to wander, stroll about" (see vagary). But on another theory the Anglo-French word ultimately is from Old French vagant, with an unetymological -r-. Middle English also had vagaunt "wandering, without fixed abode" (late 14c.), from Old French vagant.