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Henry has always led what could be called a sedentary life, if only he'd every got as far as actually sitting up.
— Henry Reed · Not a Drum was Heard (unpublished radio play, 1959)
Behind the line May contain spoilers.
Our reading
An editorial interpretation.
Henry's life is described as falling short even of sitting up. The joke stretches the idea of a sedentary existence until sitting becomes an achievement rather than ordinary inactivity. That reversal gives the sentence its dry humour, making a restrained description carry an extravagant exaggeration.
Context reviewed