I'm sure Postdog is on the right lines with the 'apprenticeship' idea above. Certainly, the times I myself most frequently heard the phrase was in the days of National Service. Conscripted men were often encouraged to serve for longer than the two years they had to. Many did, only to be told by those who chose not to that they were "signing their lives away"...ie literally committing extra chunks of their life to military service. Of course, once you are in the military, large areas of your life are out of your control.