Immemorial, unremembered, time out of mind. As proof of how we forget a post-positive adjective's meaning, here is a chap interviewed in The Guardian about a cricketer's marriage:
India and Pakistan, for time immemorial, have been portrayed as two warring nations which has had a humongous* impact on our psyche.
Now as Pakistan only became a state in 1947, that implies either a strange amnesia on the part of our pensioners, or that immemorial has slunk out of significance.
The reason for this barren yoking is that time immemorial has, or had, a precise meaning in English law. It was the time before the 6th of July 1189 and was set as such in 1276, meaning that human memory must last for 87 years, which seems about right to me.
Yet can those lovely Ms been left to rot? Shall we let Time, to almost quote Shakespeare, come and take my love away?
No. The word, though rusty, is serviceable. It is common, it's meaning is clear, and look at the results. This from Tennyson's Princess:
The moan of doves in immemorial elms,
And murmuring of innumerable bees
Count the Ms. Marvel at the mellifluous euphony. Five miles meandering with a mazy motion. It also rhymes neatly with sartorial.
She sat like Patience on a monument,
*William Hartston in The Independent quite rightly complained that humongous was “surely one of the ugliest words ever to slither its way into our dictionaries”
P.S. There is also an immemorial regiment in the Spanish Army.