1
general::
Also, time out of mind. Long ago, beyond memory or recall, as in These ruins have stood here since time immemorial, or His office has been on Madison Avenue for time out of mind. The first expression comes from English law, where it signifies "beyond legal memory," specifically before the reign of Richard I (1189-1199), fixed as the legal limit for bringing certain kinds of lawsuit. By about 1600 it was broadened to its present sense of "a very long time ago." The variant, first recorded in 1432, uses mind in the sense of "memory" or "recall.''
American Heritage Idioms