  We report the results of our study of the high-velocity (\approx160 km/s)
runaway O star CPD-64 2731 and its associated horseshoe-shaped nebula
discovered with the Wide-field Infrared Survey Explorer. Spectroscopic
observations with the Southern African Large Telescope and spectral analysis
indicate that CPD-64 2731 is a fast-rotating main-sequence O5.5 star with
enhanced surface nitrogen abundance. We derive a projected rotational velocity
of \approx300 km/s which is extremely high for this spectral type. Its
kinematic age of \approx6 Myr, assuming it was born near the Galactic plane,
exceeds its age derived from single star models by a factor of two. These
properties suggest that CPD-64 2731 is a rejuvenated and spun-up binary
product. The geometry of the nebula and the almost central location of the star
within it argue against a pure bow shock interpretation for the nebula.
Instead, we suggest that the binary interaction happened recently, thereby
creating the nebula, with a cavity blown by the current fast stellar wind. This
inference is supported by our results of 2D numerical hydrodynamic modelling.
