subsubspecies

[…]and any geographical sections of the whole which have been called by distinct names are, by definition, only subspecies, or even subsubspecies.

1898, A[dolf] B[ernhard] Meyer, L[ionel] W[illiam] Wiglesworth, The Birds of Celebes and the Neighbouring Islands, volume I, Berlin: R. Friedländer & Sohn, page 52

Cases may also be encountered where very distinct subspecies appear to have been further segregated into what might be equivalent to “subsubspecies.”

1944 November, E. Gorton Linsley, “The Naming of Infra-Specific Categories”, in Entomological News, volume LV, number 9, Philadelphia, Pa.: The American Entomological Society; The Academy of Natural Sciences, pages 227–228

Moreover, a species usually encompasses subordinate classification groups such as subspecies (or varieties), subsubspecies (or races), and strains, and these too are distinguished by particular, unique variations (Fig. 16.2).

1966, Paul B. Weisz, The Science of Zoology, McGraw-Hill Book Company, →LCCN, page 437