I don't want to say this, but
A preamble, often used, which precedes something that the speaker cannot resist the urge to say (for a variety of reasons including loose mouth).
It is also either or both of the following:
Extremely critical of the person being spoken to, Revealing the true nature of the person speaking.
It is also either or both of the following:
Extremely critical of the person being spoken to, Revealing the true nature of the person speaking.
"I don't want to say this, but you're a heartless bastard who is too eager to get into fights"