fools rush in where angels fear to tread
Definition from Wiktionary, the free dictionary
From Essay on Criticism (1711) by Alexander Pope.
- A person who does not plan ahead and think matters through becomes involved in risky or unfavorable situations which prudent people avoid.
Usage notes 
- Often abbreviated as fools rush in.
- Gregory Y. Titelman, Random House Dictionary of Popular Proverbs and Sayings, 1996, ISBN 0-679-44554-4, p. 104.