A convincing form of fruitlessness, as it’s hard to imagine a pious vulture. The sister in question is an awkward customer, not easy for her entirely reasonable brother to reason with.
‘One glance at his sister was sufficient to make him realize that to attempt reason with her would be about as fruitful as to try to christianize a vulture.’
Source: Mervyn Peake, Titus Groan, introduction by Anthony Burgess (London: Vintage Books, 1998), p. 292
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