What does Boxing Day have to do with boxing?
1839, from out + put (v.). Till c.1880, a technical term in the iron and coal trade [OED]. The verb is attested from mid-14c., originally "to expel;" meaning "to produce" is from 1858.
output out·put (out'put')n. The amount produced, ejected, or excreted by an entity during a specified time.
The energy, power, or work produced by a system or device.
The information that a computer produces by processing a specific input.