Lord Campbell, in his entertaining Lives of the _Chief Justices_, says,
in paragraph introductory to the life of Sir Edward Coke:
"As the name does not correspond very aptly with the notion of
their having come over with the Conqueror, it has been derived
from the British word 'Cock' or 'Coke' a 'Chief;' but, like
'Butler,' 'Taylor,' and other names now ennobled, it much more
probably took its origin from the occupation of the founder of the
race at the period when surnames were first adapted in England.
Even in Queen Elizabeth's reign, as well as that of James I., Sir
Edward's name was frequently spelt 'Cook.' Lady Hatton, his second
wife, who would not assume it, adopted this spelling in writing to
him, and according to this spelling, it has invariably been
pronounced."
Lord Campbell, who seems rather fond of such speculations, however, in
the case of Lord Cowper does not give the etymology of the name. But he
gives a letter written from school by the subsequent chancellor, in
which he signs his name "William Cooper." However, elsewhere, in a note
he speaks of the propensity evinced by those who have risen to wealth
and station to obliterate the trace of their origin by dropping, adding,
or altering letters and among them he mentions "Cowper" as having its
origin in "Cooper." Mr. Mark Antony Lower, too, in his _Essay on English
Surnames_, classes Cowper among the surnames derived from trade.
Possibly, therefore, notwithstanding the alteration, the original
pronunciation has been continued.