Learn English – Are the phrases “once for all” and “once and for all” equally acceptable

‘At all’ means ‘in every way, in any way’ and has been so used for around 600 years. It is now mostly found in negative and interrogative sentences. The ‘all’ element is perhaps self-explanatory. ‘At’ has many uses, not necessarily confined to the location of objects in space and time. They include the now obsolete ‘introducing the reason or consideration’ (OED), which is one possible source for its use in what must now be regarded as an idiom.

It's a reference to the prophecy that King Arthur will return. The idea is that he was once king, and will be again.

As far as I know, T.H. White did in fact coin the English version of the phrase for his Arthurian book The Once and Future King, but you'll occasionally hear it adapted for other uses ("ladies and gentleman, the once and future champion!"), presumably as an allusion to the book. The original source is Sir Thomas Malory's Le Morte d'Arthur (the most enduringly popular rendition of the Arthurian myth), where the equivalent Latin phrase rex quondam rexque futurus is described as engraved on Arthur's tombstone.