In most compound motors the shunt winding is the main field winding but is possible to wind the motor so that the series field provides most of the field and the shunt winding contributes a smaller proportion. From an old reference book I think this latter case is what is being considered, i.e. a series motor with a weak shunt field as opposed to a shunt motor with a weak series field which is the more common variety of compound motor.
It is also possible to wind the series field in opposition to the shunt field in which case it will have a destabilising effect on the motor in terms of speed regulation.