unfattenable

From un- +‎ fatten +‎ -able.

unfattenable (comparative more unfattenable, superlative most unfattenable)

1. That cannot be fattened.
   • 1854, William Graham, The Jordan and the Rhine; or, The East and the West, page 46:

     The cow is a large, raw, high-boned, unfattenable animal, and being regularly wrought in the cultivation of the soil the milk is bad.

   • 1963, Home Economics, volume 9, page 6:

     Of the many explanations that have been put forward one is that the unfattenable person consumes the calories by much greater muscular energy than the fattenable person.