• Webster's Revised Unabridged Dictionary
    • v. i Disimprove To grow worse; to deteriorate. "Every face . . . worsting ."
    • v. t Disimprove To make worse; -- the opposite of improve.
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Century Dictionary and Cyclopedia
    • disimprove To render worse; injure the quality of.
    • disimprove To grow worse.
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Chambers's Twentieth Century Dictionary
    • v.t., v.i Disimprove dis-im-prōōv′ to render worse, to grow worse.
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In literature:

I don't think you will find Valetta disimproved by her crisis.
"Beechcroft at Rockstone" by Charlotte M. Yonge