It is a term used in early 20th century continental philosophy, especially in the works of Martin Heidegger, Jean-Paul Sartre, Simone de Beauvoir, and the existentialists.
(Heidegger 1962, p. H.78) The argument for this claim draws heavily on Hegel's great work, the Phenomenology of Spirit.
Despite Heidegger's interest in it, he returns — more often than not indirectly — to the subject by interrogating other concepts that simply invoke Being without explicitly acknowledging it.
This mode of being is relevant to inanimate objects, but not to humans, who Sartre says must always make a choice.
He is primarily a man (being-for-itself), just one who happens to be functioning as a waiter – with no fixed nature or essence, who is constantly recreating himself.