  We report a Giant Metrewave Radio Telescope HI 21cm mapping study of the
neutral atomic hydrogen (HI) in the host galaxy of the fast radio burst (FRB)
FRB20180916B at $z \approx 0.03399$. We find that the FRB host has an HI mass
of $\rm M_{HI} = (2.74 \pm 0.33) \times 10^9 \ M_\odot$ and a high
HI-to-stellar mass ratio, $\approx 1.3$. The FRB host is thus a gas-rich but
near-quiescent galaxy, that is likely to have acquired a significant mass of HI
in the recent past. The HI distribution is disturbed, with extended HI 21cm
emission detected in a north-eastern tail, a counter-tail towards the south, an
HI hole between the galaxy centre and the FRB location, and a high HI column
density measured close to the FRB position. The FRB host is part of a group
with four companions detected in their HI 21cm emission, the nearest of which
is only 22~kpc from the FRB location. The gas-richness and disturbed HI
distribution indicate that the FRB host has recently undergone a minor merger,
which increased its HI mass, disturbed the HI in the galaxy disk, and
compressed the HI near the FRB location to increase its surface density. We
propose that this merger caused the burst of star-formation in the outskirts of
the galaxy that gave rise to the FRB progenitor. The evidence for a minor
merger is consistent with scenarios in which the FRB progenitor is a massive
star, formed due to the merger event.
