The Collaborative International Dictionary of English v.0.48:

Headwork \Head"work`\ (h[e^]d"w[^u]rk`), n.
   Mental labor.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "headwork":
   abstract thought, act of thought, application, boning, brainwork,
   cerebration, cogitation, conceit, conception, conceptualization,
   conning, contemplation, cram, cramming, creative thought, drill,
   engrossment, excogitation, exercise, extensive study, grind,
   grinding, heavy thinking, idea, ideation, imageless thought,
   inspection, intellection, intellectual exercise,
   intellectualization, lucubration, mental act, mental labor,
   mental process, mentation, noesis, perusal, practice,
   ratiocination, reading, reasoning, restudy, restudying, review,
   straight thinking, study, studying, subject, swotting, thinking,
   thinking aloud, thinking out, thought, wide reading