You’re extremely unlikely to encounter this old adjective relating to yesterday, it being one of the rarest in the language.
The Oxford English Dictionary has only two examples, one from a glossary of 1656, and my electronic searches have failed to find any more beyond the OED’s other citation, which is from William Makepeace Thackeray’s A Shabby Genteel Story of 1840: “Thrice a-week, at least, does Gann breakfast in bed — sure sign of pridian intoxication”.
It has the most respectable antecedents — it’s from Latin pri-, before, plus dies, day, and so belongs with diary, diurnal, journal, and journey, all of which can likewise be traced back to dies.
However, like an ineffectual political candidate, it was unable to muster enough support to be elected a permanent member of the English lexicon, and we must now consider it to be one of yesterday’s words.
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