  We provide a geometric realization of the quasi-split affine $\imath$quantum
group of type AIII$_{2n-1}^{(\tau)}$ in terms of equivariant K-groups of
non-connected Steinberg varieties of type C. This uses a new Drinfeld type
presentation of this affine $\imath$quantum group which admits very nontrivial
Serre relations. We then construct \`a la Springer a family of
finite-dimensional standard modules and irreducible modules of this
$\imath$quantum group, and provide a composition multiplicity formula of the
standard modules.
