That's "ipsa." Singular, to agree with "res." (Someone has to uphold standards around here.)
Your're right, I was thinking of "Res ipsa loquitur".
(with a long a in ipsa, IIRC. The thing, for itself, speaks as opposed to the thing itself speaks)
From the Wikipedia article:
"Res ipsa loquitur, sed quid in infernos dicet?"
("The thing speaks for itself, but what the hell is it saying?")
[I ran the phrase I posted through a Latin translator, and it doesn't even appear to be well-formed!]