E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Head of Cattle.
Cattle are counted by the head; manufacturing labourers by hands, as How many hands do you employ? horses by the nose (See NOSE); guests at dinner by the cover, as Covers for ten, etc. (See NUMBERS, HAND.)
In contracting for meals the contractor takes the job at so much a headi.e. for each person.