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We present a high-resolution ($R\sim35,000$), high signal-to-noise($S/N=350$) Magellan/MIKE spectrum of the bright extremely metal-poor star2MASS~J1808$-$5104. We find [Fe/H] = $-$4.01 (spectroscopic LTE stellarparameters), [Fe/H] = $-$3.8 (photometric stellar parameters), [Fe/H] = $-$3.7(spectroscopic NLTE stellar parameters). We measured a carbon-to-iron ratio of$\mbox{[C/Fe]}= 0.38$ from the CH G-band. J1808$-$5104 is thus notcarbon-enhanced, contrary to many other stars with similarly low ironabundances. We also determine, for the first time, a barium abundance($\mbox{[Ba/Fe]} =-0.78$), and obtain a significantly reduced upper limit forthe nitrogen abundance ([N/Fe]$ < - 0.2$). J1808$-$5104 has low ratio of$\mbox{[Sr/Ba]}=-0.17$, which is consistent with that of stars in ultra-faintdwarf galaxies. We also fit the abundance pattern of J1808$-$5104 withnucleosynthesis yields from a grid of Population\,III supernova models. Thereis a good fit to the abundance pattern which suggests J1808$-$5104 originatedfrom gas enriched by a single massive supernova with a high explosion energy ofE $=10\times10^{51}$\,erg and a progenitor stellar mass ofM$=29.5$\,M$_{\odot}$. Interestingly, J1808$-$5104 is a member of the Galacticthin disk, as confirmed by our detailed kinematic analysis and calculatedstellar actions and velocities. Finally, we also established the orbitalhistory of J1808$-$5104 using our time-dependent Galactic potential the\texttt{ORIENT}. J1808$-$5104 appears to have a stable quasi-circular orbit andbeen largely confined to the thin disk. This unique orbital history, the star'svery old age ($\sim13.5$\,Gyr), and the low [C/Fe] and [Sr/Ba] ratios suggestthat J1808$-$5104 may have formed at the earliest epoch of the hierarchicalassembly of the Milky Way, and it is most likely associated with the primordialthin disk.

The chemical abundance pattern of the extremely metal-poor thin disk star 2MASS J1808-5104 and its origins