The Collaborative International Dictionary of English v.0.48:

Overtax \O`ver*tax"\, v. t.
   To tax or to task too heavily; as, a job that overtaxed his
   physical energies.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

overtax
    v 1: tax excessively; "Don't overtax my constituents!"

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "overtax":
   bear hard upon, bleed, bleed white, burden, burthen, busy,
   carry too far, charge, clip, cumber, drive, encumber, exaggerate,
   exhaust, exploit, fag, fleece, freight, go overboard,
   go to extremes, go too far, gouge, hamper, handicap, haul, heave,
   hold up, keep busy, know no bounds, lade, load, oppress, overact,
   overburden, overcarry, overcharge, overdevelop, overdo, overdrive,
   overelaborate, overemphasize, overexercise, overexert, overexpend,
   overextend, overindulge, overlabor, overlade, overload, overplay,
   overprice, overreact, overspend, overstrain, overstress, overstudy,
   overtask, overtrain, overuse, overweigh, overweight, overwork,
   pass all bounds, press, press hard upon, profiteer,
   protest too much, pull, rack, rest hard upon, saddle, screw, skin,
   soak, stick, sting, strain, strain every nerve, stress, stretch,
   surcharge, sweat, sweat blood, swindle, task, tax, tense, tug,
   victimize, weigh heavy on, weigh on, weigh upon, work