Green’s Dictionary of Slang

a dishonest drover or shepherd who connives at stealing the cattle they are guarding.

1538		T. Elyot Dict. n.p.: Abactores, theues that steal cattell.
1556		J. Withals Dictionarie in Eng. and Latine Qiii: Abactor, he that stealeth catell.
1565		T. Cooper Thes. Linguae Romanae et Britannicae n.p.: Abactor, abactôris, m.g. Verbale. Apul. A theefe: a stealer of cattell or beastes.
1659		H. Hammond On Psalms in Farmer & Henley Sl. and Its Analogues I (1890–1904) 3/1: Invaders and abactors, whose breaking in...is attended with the cattels passing through or going out .
1691		T. Blount Law Dict. n.p.: Abactors: were stealers of Cattle or Beasts, by Herds, or great numbers; and were distinguished from Fures.
1818		Annual Register [abridged] in Farmer & Henley Sl. and Its Analogues I (1890–1904) 3/1: One of the tricks of the abacters of old Smithfield was the driving of a bullock into a jeweller’s or other shop, and during the confusion the abacter’s confederates would help themselves to any valuables handy...one shop was so served three times in the year .
1829		C. Lamb in Correspondence with Procter/ Works II (1870) 23: I thought, if she went no more, the Abactor or Abactor’s wife [...] would suppose she had heard something; and I have delicacy for a sheep-stealer.