  Ferroelectric quantum spin Hall insulator (FEQSHI) exhibits coexisting
ferroelectricity and time-reversal symmetry protected edge states, holding
fascinating prospects for inviting both scientific and application advances,
especially in two dimensions. However, all of the previously demonstrated
FEQSHIs consist two or more constituent elements. We herein propose the
$\psi$-bismuthene, an uncharted allotrope of bilayer Bi (110), to be the first
example of 2D elemental FEQSHI. It is demonstrated that $\psi$-bismuthene
harbors measurable ferroelectric polarization and nontrivial band gap with
moderate switching barrier, which are highly beneficial for the detection and
observation of the ferroelectric topologically insulating states. In addition,
all-angle auxetic behavior with giant negative Poisson's ratio and
ferroelectric controllable persistent spin helix in $\psi$-bismuthene are also
discussed. The emergent elemental FEQSHI represents a novel domain for both
fundamental physics and technological innovation.
