sure as death en · ADV
Meanings
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(not-comparable, qualifier:simile) Most certainly.
"It was naething to make a wark about. It was just yon windy yonder in the library that is nae windy. And it was open as sure's death. You may laugh if you like. Is that a' she's wantin' wi' me?"
1896, Margaret Oliphant, The Library Window: