Almost never appears alone in modern Chinese — you'll see it locked into set phrases: 居心叵测 (intentions unfathomable/sinister), 叵耐 (intolerable, classical novels), 叵测 (unfathomable).
memory
叵 is literally 可 ("can") written backwards — it's the negation of 可, so 叵 = "cannot." Visualizing it as a mirrored 可 makes the meaning click.
