  Extended 21cm absorption regions (dubbed ``21cm absorption halos'') around
first galaxies at $z\sim 30$ are likely the first distinctive structures
accessible to radio observations. Though the radio array capable of detecting
and resolving them must have $\sim 200$ km$^2$ total collecting area, given the
great impact of such detections to the understanding of the reionization
process and cosmology, such radio survey would be extremely profitable. As an
example, we point out a potentially useful byproduct of such survey. The
resolved 21cm absorption ``halos'', likely close to spherical, can serve as
(almost) ideal sources for measuring the {\it cosmic shear} and mapping the
matter distribution to $z\sim 30$. We investigate the expected lensing signal
and consider a variety of noise contributions on the shear measurement. We find
that S/N $\sim 1$ can be achieved for individual ``halos''. Given millions of
21cm absorption ``halos'' across the sky, the total S/N will be comparable to
traditional shear measurement of $\sim$$10^9$ galaxies at $z\sim 1$.
