To blurt or blurt out.
from Wiktionary:
blurt [Verb]:
To utter suddenly and unadvisedly; to speak quickly or without thought; to divulge inconsiderately — commonly with out.
- Please think about your reply and don't just blurt out the first thing that comes to mind.
Further to comments to the OP:
- to "put your foot in your mouth" is to say something embarrassing, and
- to "spill the beans" is to tell the facts about something (presumably intentionally).
- To let something slip is to unintentionally reveal a fact which you should have kept secret — is this the meaning you wanted?
