dead·head 
(dĕdhĕd′)
(dĕdhĕd′)n.
1. A person who uses a free ticket for admittance, accommodation, or entertainment.
2. A vehicle, such as an aircraft, that transports no passengers or freight during a trip.
3. Informal A person regarded as dull-witted or sluggish.
4. A log or trunk that is partially submerged or lying just beneath the surface.
v. dead·head·ed, dead·head·ing, dead·heads
v.tr.
1. To pilot or drive (a vehicle) carrying no passengers or freight.
2. To remove dead or withered flowers from (a plant), especially to promote new blooms or prevent the setting of seeds.
v.intr.
1.
a. To travel as an employee of an airline or other public carrier for free, especially en route to or from one's assigned duties.
b. To make a trip without passengers or freight.
c. To pilot or drive a vehicle on such a trip: "The instruments were out, and it meant they had to deadhead back on another airplane" (Walter J. Boyne).
2. To bypass a senior employee in order to promote a more junior employee.
adv.
Without passengers or freight; empty.