Alienation, which in German is Entfremdung, is to make oneself feel estranged about something – and that can include one’s very own Dasein, such that, to use a common psychological example, one is overcome by a feeling of strangeness when, looking at oneself in the mirror, what is there is the Stimmung of not feeling quite at home with oneself: what Heidegger vividly describes in section 40 of Being and time as das Unheimliche. Its standard translation is “the uncanny”. As the negation of familiarity, comfort and security, the uncanny thrusts Dasein into a confrontation with “the nothing”, das Nichts, which unsettles in its nothingness (Nichtigkeit). In these uncomfortable, if not distressing, moments, Dasein becomes clearly aware of a fundamental aspect of being, which is its staying away from ground as Ab-grund, exemplified in the primordial event of a being like Dasein clearing itself authentically in an uncanny gap in the uneven ecstasis of existential time. Geworfenheit,which belongs to Dasein‘s temporalisation (Zeitigung), becomes the free fall of Dasein into the absence of everyday understanding of itself and of existence, where time bends and ceases to become an even line of now-points (Jetztpunkte). The next moment in Dasein‘s existence is now filled with uncertainty, which is not just another “now” and is the condition of Dasein‘s Angst for its inescapable mode of being as being-in-the-world. In short, through the uncanny, Dasein enters the meaningful crisis of a deep attunement to the world, which the illusory protection of everyday “idle talk” (Gerede) in the inauthentic company of “the they” (das Man) shields Dasein from.
One of the central teachings of Being and time is the disclosedness (Erschloßenheit) of Dasein as being-toward-death (Sein zum Tode) in its being-in-the-world. The ultimate horizon of Dasein‘s temporal (and temporalised) existence is death. The uncanny unsettles in the form of Angst because in the final analysis, death comes into play. The fulfilment of Dasein‘s potentiality-of-being (Seinkönnen), i.e. fullness in its ending as indicated in the German word for perfection as Vollendung, becomes a vulnerable directionality (Ausrichtung) in the ecstases of time as Dasein‘s understanding of death coincides with its discovery, through the authenticity of its Angst, the abyssal nature of being, which Dasein, as being-toward-death, itself is.