"next in order after the tenth; an ordinal numeral; being one of eleven equal parts into which a whole is regarded as divided;" late 14c., eleventhe, superseding earlier ellefte (c. 1300), enlefte (early 13c.), from Old English endleofta; see eleven + -th (1). Eleventh hour "last moment, just before it is too late" is in Old English, from the parable of the laborers in the vineyard (Matthew xx.1-16); as an adjective by 1829.
But aboute the elleventhe hour he wente out and founde other stondynge, and he seide to hem, what stonden ye idel heere al dai? [Wyclif, Matthew xx]