A gentleman is one who never hurts anyone’s feelings unintentionally.

Oliver Herford (1863–1935) Anglo-American writer, artist and illustrator

(Attributed)

Widely attributed to Herford as early as 1915. Similar in construction to "A gentleman is a man who never gives offense unintentionally," "A gentleman is a person who never insults anyone unintentionally," etc., which go back to at least 1905, and in the late 1920s were attributed without citation to Oscar Wilde. See here for more discussion of the Wilde citation.

By possible coincidence, Herford was known at the time as the American Oscar Wilde.