1610s, "a calling away from one's occupation;" 1640s, "that which calls one away from one's proper business," from Latin avocationem (nominative avocatio) "a calling away, distraction, diversion," noun of action from past-participle stem of avocare "to call off, call away," from ab "off, away from" (see ab-) + vocare "to call," which is related to vox (genitive vocis) "voice" (from PIE root *wekw- "to speak"). Commonly, but improperly, "one's regular business, vocation" (1660s). Earlier (1520s) in a legalistic sense "calling to a higher court."
But yield who will to their separation,
My object in living is to unite
My avocation and my vocation
As my two eyes make one in sight.
[Frost, "Two Tramps in Mud Time"]