One term for an underlying, primordial belief is ur-belief. You can get a sense of how people use it from the following excerpts. From Barbara Ardinger, Pagan Everyday: Finding the Extraordinary in Our Oridnary Lives (2008):
But everyone has heard of the man in the moon. He first appeared in a Saxon folk tale with his wife, the woman in the sun. To our European ancestors, the sun was female and the moon was male. Earlier, the Sumerian city Ur (we use this name [ur] to indicate the oldest anything, like an ur-belief) was named after the moon god Hur. He's an ur-god.
From Michael Jacobsen & Keith Tester, eds., Utopia: Social Theory and the Future (2012):
In other words, Islamism is neither the manifestation of some Islamic Ur-belief nor the attempt to revive traditional Islamic practice, but rather a modern political ideology whose main target is the world's growing economic and political liberalization with its 'anarchic' and 'alienating' effects (individualism, toleration, promiscuity, women's and gay rights, consumerism, commercialism, and so forth).
From David Lynch: The Air Is on Fire (2007) [combined snippets]:
It was the kind of art brought by those artists emigrating to America in the 1 930s and 1 940s from a Europe threatened by the Nazis. They brought an insight into both the fragility and the precariousness of human nature. And this insight was something which contradicted the optimism prevalent in America at the time, a particular ur-belief in the immanent goodness of nature intact — human nature included — and the possibility of finding earthly happiness. The expressionist-surrealist tradition of modern art viewed man as a creature involved in a fundamental conflict with himself and the world.
From Stellan Ohksson, Deep Learning: How the Mind Overrides Experience (2011):
This Janus-faced character of belief systems does not imply that a belief base branches out from a single starting point, a single Ur-belief. A more likely view is that a belief base is anchored in multiple parentless nodes, representing the person's most fundamental beliefs about material reality, life, society and so on.
And from Zina Giannopoulou, Plato's Theatetus as a Second Apology (2013):
In the current context, this appeal [to Protagoras' own statement] is cashed out as an attempt to provide reasons for Protagoras' second—order beliefs, even if these reasons are—and, given the sophist's absence from the dialectical proceedings, can be no more than—restatements of (P), Protagoras' ur—belief, as it were.
The term ur-prejudice functions similarly. From E. Joseph Sharkey, Idling the engine: Linguistic Skepticism in and Around Cortázar, Kafka, and Joyce (2006):
That Oliveira applies the sovereignty of the aesthetic consciousness to language shows the severity of his self-consciousness: he distances himself even from the medium of his own understanding. Because “[u]nderstanding is language-bound” (PH, 15), language can be understood as the ur-prejudice, the prejudice fundamental to all understanding. “Language is not one of the means by which consciousness is mediated with the world," writes Gadamer. "Rather, in all our knowledge of ourselves and in all knowledge of the world, we are always already encompassed by the language that is our own" (PH, 62).
Michael Quinion, Ologies and Isms: Word Beginnings and Endings (2002) has this to say about the prefix ur-:
ur- Primitive; original; earliest. [Frpm Grman.]
An urtext is an original or the earliest version of a text, to which later versions can be compared; an ur-novel is the earliest example of a novel in modern form, or one written in a deliberately primitive style; an ur-language (often also Ur-sprache from German) is a hypothetically reconstructed parent language.
So... are ur-belief and ur-prejudice the terms you were trying to recollect (in the Socratic sense)?