  Stellar counter-rotation in disk galaxies directly relates to the complex
phenomenon of the disk mass assembly believed to be driven by external
processes, such as accretion and mergers. The detailed study of such systems
makes it possible to reveal the source of external accretion and establish the
details of this process. In this paper, we investigate the galaxy PGC 66551
(MaNGA ID 1-179561) which hosts two large-scale counter-rotating (CR) stellar
disks identified in the Sloan Digital Sky Survey MaNGA data and then confirmed
using deep follow-up spectroscopy with the 10m Southern African Large
Telescope. We measured the properties of ionized gas and stellar populations of
both CR disks in PGC 66551. We found that the CR disk is compact, contains
young stars with subsolar metallicity, and has a stellar mass of
$5\times10^{9}$ M$_\odot$ which amounts to $\approx$20% of the galaxy's total.
Surprisingly, the main 8 Gyr old disk has a significantly lower metallicity of
-0.8 dex than other CR galaxies. We developed a simple analytic model of the
history of the metal enrichment, which we applied to PGC 66551 and constrained
the parameters of the galactic outflow wind, and estimated the metallicity of
the infalling gas that formed the CR disk to be -0.9 ... -0.5 dex. Our
interpretation prefers a merger with a gas-rich satellite over cold accretion
from a cosmic filament as a source of gas, which then formed the CR disk in PGC
66551.
