  The Hobby-Eberly Telescope is used to measure the velocity dispersion profile
of the nearest prototypical cD galaxy, NGC 6166 in cluster Abell 2199. We also
present surface photometry from many telescopes. We confirm the defining
feature of a cD -- a halo of stars that fills the cluster center and that is
controlled by cluster gravity, not by the central galaxy. The velocity
dispersion of NGC 6166 rises from 300 km/s at the center to 865 +- 58 km/s at
100 arcsec radius in the halo. This shows for the first time that the
dispersion rises all the way to the cluster value, 819 +- 32 km/s. We find that
the main body of NGC 6166 moves at 206 +- 39 km/s with respect to the cluster
velocity, whereas the velocity of the cD halo is 70 km/s closer to the cluster
velocity. These results support our picture that cD halos consist of stars that
are stripped from cluster galaxies. But we do not confirm the view that cD
halos are an extra, low-surface-brightness component that is distinct from the
main body of a normal giant elliptical. Instead, all of the brightness profile
of NGC 6166 outside its core is described to +- 0.037 mag/arcsec**2 by a single
Sersic function with index n ~ 8.3. The cD halo is not recognizable from
photometry alone. This blurs the distinction between cDs and similar-n
core-boxy-nonrotating ellipticals. Both may have halos made largely via minor
mergers and the accumulation of debris. However, the cD halo of NGC 6166 is as
enhanced in alpha elements as the main body. Quenching of star formation in <~1
Gyr happened even to the galaxies that contributed the cD halo.
