Someone happened to use the phrase "the 59th minute of the eleventh hour" just now on IRC (#lisp on Freenode). I remarked that that should be "the twelfth hour". This then started me wondering where that apparently nonsensical phrase came from. It means at the last minute, of course, but in what sense is the eleventh hour the last minute? A quick google finds, for example http://en.wiktionary.org/wiki/eleventh_hour, but that doesn't really explain anything. The Biblical usage seems to mean "late in the day" (apparently 5 pm or so), not "at the last minute". Or is it just that the meaning of the phrase has changed slightly over time?
Its origin is biblical and refers to the parable of the labourers in the vineyard (Matthew XX)
The OED gives some indication of the etymology of the modern expression from as early as 971AD.
eleventh hour: the latest possible time, in allusion to the parable of the labourers ( Matt. xx.); also eleventh-hour used attrib. or as adj.
971 Blickl. Hom. 93 Eall eorþe bið mid þeostrum oforþeaht æt þa endlyftan tid þæs dæges.