late 14c., conservatyf, from Middle French conservatif, from Late Latin conservativus, from Latin conservatus, past participle of conservare (see conserve).
As a modern political tradition, conservatism traces to Edmund Burke's opposition to the French Revolution (1790), but the word conservative is not found in his writing. It was coined by his French disciples, (e.g. Chateaubriand, who titled his journal defending clerical and political restoration "Le Conservateur").
Conservative as the name of a British political faction first appeared in an 1830 issue of the "Quarterly Review," in an unsigned article sometimes attributed to John Wilson Croker. It replaced Tory (q.v.) by 1843, reflecting both a change from the pejorative name (in use for 150 years) and repudiation of some reactionary policies. Extended to similar spirits in other parties from 1845.
Phrases such as a conservative estimate make no sense etymologically. The noun is attested from 1831, originally in the British political sense.