We report a deep Giant Metrewave Radio Telescope (GMRT) search for Galactic H{sc i} 21cm absorption towards the quasar B0438$-$436, yielding the detection of wide, weak H{sc i} 21cm absorption, with a velocity-integrated H{sc i} 21cm optical depth of $0.0188 pm 0.0036$~km~s$^{-1}$. Comparing this with the H{sc i} column density measured in the Parkes Galactic All-Sky Survey gives a column density-weighted harmonic mean spin temperature of $3760 pm 365$~K, one of the highest measured in the Galaxy. This is consistent with most of the H{sc i} along the sightline arising in the stable warm neutral medium (WNM). The low peak H{sc i} 21cm optical depth towards B0438$-$436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the H{sc i} 21cm absorption and emission spectra. This yields a gas kinetic temperature of $rm T_k leq (4910 pm 1900)$~K, and a spin temperature of $rm T_s = (1000 pm 345)$~K for the gas that gives rise to the H{sc i} 21cm absorption. Our data are consistent with the H{sc i} 21cm absorption arising from either the stable WNM, with $rm T_s ll T_k$, $rm T_k approx 5000$~K, and little penetration of the background Lyman-$alpha$ radiation field into the neutral hydrogen, or from the unstable neutral medium, with $rm T_s approx T_k approx 1000;K$.
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