the GNU version of the Collaborative International Dictionary of English
v. To print in Italic characters; to underline written letters or words with a single line
The Century Dictionary and Cyclopedia
To print in italic type, or underscore with a single line in writing: as, to italicize emphatic words or sentences; in old books all names were commonly italicized. Also spelled italicise.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
v. print in italics
Word Usage
"The standard instruction to typesetters to italicize a word in a handwritten manuscript or a proof was to draw a line underneath it."