To develop an illness or sustain an inury, possibly self inflicted, which will preclude you from work, but you can still enjoy a rich and varied social life.
Orginates from the 1914-18 War, when reluctant English soldiers would shoot themselves in the foot, to get returned to Blighty, or Great Britan, and invalided out of the Army.
Hangovers can count if passed of as food poisoning
-I intend to get so drunk tonight I will phone in sick tomorrow
-Oh really, are you about to Take a Blighty?