pre-noun adjectival
feigned; pretending not to know
Used almost exclusively in fixed expressions like 素知らぬ顔 (feigned ignorance face) or 素知らぬ振り (pretending not to know). Describes acting as if one is unaware of something.
彼は私の失敗に気づいているのに、素知らぬ顔をしている。
He knows about my mistake, but he's putting on a face of feigned ignorance.
彼女は素知らぬ振りでその話題を避けた。
She avoided the topic by pretending not to know.
知らんぷり is a noun/suru-verb meaning 'feigning ignorance' and can be used more broadly, while 素知らぬ is a pre-noun adjectival that directly modifies nouns like 顔 or 振り.
The derivation is uncertain. The first element 素 (そ) may be related to the prefix meaning 'plain' or 'mere', and 知らぬ is the classical negative form of 知る (to know), giving a literal sense of 'not knowing plainly'.