Indeed It is surely a word of noble Messenger.
Since Messenger was illiterate, the disbelievers were doubting and were making allegation that what Muhammed was telling them as divine Message-Adhikr-was because of Jinn affected on him. So through this verse it is told them that Adhikr is the word of a noble Messenger. That means Muhammed who was known as Al Ameen-the honest-is a noble Messenger from Lord who does not tell any lie, and you need not wonder or doubt about his honesty.
Since Messenger Muhammed reflected Adhikr which is mentioned in verses 56: 77-79 as a Noble reading from a well-guarded Book, and none can conceive It except the purified, Muhammed is mentioned as a noble Messenger in this verse. Since Angel Jibreel brings noble Reading Adhikr, he is mentioned as noble Messenger in verse 81: 19. See explanation 26: 192-194; 44: 17; and 53: 1-4.