  An over-dense wake is created by a gravitating object moving through a
gaseous medium, and this wake pulls back on the object and slows it down. This
is conventional dynamical friction in a gaseous medium. We argue that if the
object drives a sufficiently powerful outflow, the wake is destroyed and
instead an extended under-dense region is created behind the object. In this
case the overall gravitational force is applied in the direction of the
object's motion, producing a negative dynamical friction (NDF). Black holes in
dense gas drive powerful outflows and may experience the NDF, although
extensive numerical work is probably needed to demonstrate or refute this
conclusively. NDF may be important for stellar-mass black holes and neutron
stars inside `common envelopes' in binary systems, for stellar mass black holes
inside AGN discs, or for massive black holes growing through super-Eddington
accretion in early Universe.
