Kural - 845

If men what they have never learned assume to know,
Upon their real learning's power a doubt 'twill throw

Explanation
Fools pretending to know what has not been read (by them) will rouse suspicion even as to what they have thoroughly mastered.

Tamil Transliteration
Kallaadha Merkon Tozhukal Kasatara
Valladhooum Aiyam Tharum.