The Collaborative International Dictionary of English v.0.48:

Italicize \I*tal"i*cize\, v. t. & i. [imp. & p. p. Italicized;
   p. pr. & vb. n. Italicizing.]
   To print in Italic characters; to underline written letters
   or words with a single line; as, to Italicize a word;
   Italicizes too much.
   [1913 Webster]