Beau's lines

noun plural

ˈbōz- 

: transverse grooves or ridges on a fingernail or toenail that occur from a temporary disruption in nail growth (such as that resulting from severe illness, malnutrition, or chemotherapy)

The nails of someone who had pneumonia several weeks previously may show transverse grooves—these "Beau's lines" attest to the nutritional insult to the "nail plate" (the growing part, under the cuticle).

—John Stone, The New York Times Magazine