as sin en · ADV
Etymology
Generalization of previous similes, e.g. evil as sin, common as sin, imputable as sin.
Meanings
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(postpositional) To a great extent or degree; very.
As for Janie, she stood there, petrified, and now her face had become as ugly as sin, and as vicious, and as malevolent.
1945, Taylor Caldwell, Wide House:The clouds covered a liquid surface black as sin.
2015, John C. Wright, The Architect of Aeons, 1st edition, New York:There is no way you'll lift it yourself, it's heavy as sin.
CAPTAIN: I understand, and I'll patronize you. I'll take every ticket, and pay you cash, and Jedidah shall come and see the beautiful rival. / JEDIDAH: Beautiful! She's as homely as sin.
1864, O. E. [Oliver Everett] Durivage, The Stage-Struck Yankee: