  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 $\rm T_s \approx T_k
\approx 1000\;K$.
