This etymology is missing or incomplete. Please add to it, or discuss it at the Etymology scriptorium. Particularly: “Sino-Vietnamese word from 不(“not”) and 倚; 奇; 其(“to depend on; odd; it/one/that”). This term is present in the Dictionarium Anamitico-Latinum as 不期 inopinatum (unexpected). Perhaps this adjective sense became "random" and the determiner sense was gained as contamination from bất cứ.”