When promiscuous first appeared in print around the beginning of the 17th century, it meant "consisting of a heterogeneous or haphazard mixture of persons or things; composed of all sorts and conditions." That's the sense playwright Ben Jonson intended in his 1603 play Sejanus: His Fall, in which Tiberius, in Act I, asserts "Yea, even Augustus' name/Would early vanish, should it be profaned/With such promiscuous flatteries."

Perhaps appropriately, the adjective's meaning broadened and picked up the sense meaning "not restricted to one class, sort, or person; indiscriminate"; and, more specifically, "not restricted to one sexual partner." This is the sense wildlife biologist Reuben Trippensee used when he described gray squirrels as "promiscuous in their breeding habits."

Most recently, promiscuous has developed a sense synonymous with "casual; careless; irregular; random." This usage is widespread in the computer world, where promiscuous mode refers to a configuration where a network card passes all traffic it receives to the central processing unit.