|Definition - A catch-all term for any case other than the nominative and vocative (and sometimes the accusative). For example, the genitive, dative, and ablative cases are oblique cases.|
Oxford English Dictionary - The term's first citation is from 1530:
"Pronownes … have but thre cases, nominatyve, accusatyve and oblique, as, je, me, moy."
(Jehan Palsgrave, Lesclarcissement de la langue françoyse, Introduction, 30)
Quotation - "An oblique case (Latin: casus generalis) in linguistics is a noun case of synthetic languages that is used generally when a noun is the object of a sentence or a preposition. An oblique case can appear in any case relationship except the nominative case of a sentence subject or the vocative case of direct."
(Source: Wikipedia s.v. oblique case)