Costano "member of an Ohlone people" (taken to be an alteration of American Spanish costeño "coastal dweller") + -an entry 1; costeño, from costa "coast, shore," variant (with -o- probably from parallel Iberian Romance forms) of cuesta "slope, (in plural) back" (going back to Latin costa "rib, (in plural) side, flank, back") + -eño, suffix forming nouns and adjectives from place names (going back to Latin -ignus, -egnus, apparently extracted from adjectives formed with -n- in which g was part of the root, as larignus "of larch," salignus "of willow") — more at coast entry 1
The name Costano for an Ohlone group was introduced in Henry Schoolcraft's Information Respecting the History, Conditions and Prospects of the Indian Tribes of the United States, Part II (Philadelphia, 1852), p. 506, heading a word list whose compilation is credited to an "Indian agent" named Adam Johnston (see note at ohlone). Costano was reproduced by the philologist Robert Gordon Latham in two essays "On the Languages of New California" (Proceedings of the Philological Society, vol. 6, no. 134, May 13, 1853, pp. 72-86) and "On the languages of northern, western, and central America," which attempts to classify Indigenous American languages ("Read May the 9th," Transactions of the Philological Society, 1856, pp. 57). The name Costanoan with the suffix -an in reference to the language family was introduced by John Wesley Powell in "Indian Linguistic Families North of Mexico," Seventh Annual Report of the Bureau of Ethnology to the Smithsonian Institution, 1885-'86 (Washington, D.C., 1891), pp. 70-71.