。
She eyed the stranger suspiciously.
Literal
She [topic-は] that unfamiliar-person [object-を] suspicious-seemingly looked.
胡散臭い literally pairs 胡散 — an old word meaning 'suspicious' or 'shady' — with 臭い ('smelly'), painting suspicion as something with a recognizable smell. English reaches for the same metaphor when something is 'fishy'. 見知らぬ uses the classical negative ぬ as a still-productive attributive form, fused with 見知る ('to know by sight') to mean 'unfamiliar' — sister phrases like 思わぬ ('unexpected') and 知らぬ間に ('without realizing') keep this old grammar alive in everyday Japanese. The そうに ending takes the i-adjective 胡散臭い, swaps い for そう, and tacks on に to form an apparent-manner adverb modifying 見た — 'looked in a manner that seemed suspicious'.