at death's door - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
On the point of dying, very ill, as in Whenever she had a bad cold she acted as though she were at death's door. The association of death with an entry way was first made in English in the late 1300s, and the phrase itself dates from the mid-1500s. Today it is often used as an exaggeration of ill health.