1. [adj]  not intentional; "an unintended slight"; "an unintentional pun"; "the offense was unintentional"; "an unwitting mistake may be overlooked"
2. [adj]  not done or made or performed with purpose or intent
3. [adj]  without deliberate intent; "my heart with unwilled love grew warm"- George Macdonald

UNINTENTIONAL is a 13 letter word that starts with U.