afraid, bashful
葸 centers on a timid, hesitant state of mind: being afraid or bashful, shrinking back from social or uncertain situations.
葸 combines 艹 (grass) with 思 (think), but the historical development is uncertain. It may have originally expressed a fearful, shrinking feeling, but the exact origin is unclear.
Picture grass 艹 covering a thinking mind 思, making it hide away bashfully. The mind under the grass is too afraid to come out.
For サイ, imagine a shy sigh (サイ) escaping from the grass-hidden mind: sigh -> サイ, and the timid feeling makes you sigh.