What Does “Shī- (しーっ)” Mean?
👉 An onomatopoeia primarily used as an interjection to command or request immediate silence, often accompanied by a finger-to-lips gesture.
👉 To compel or urge the maintenance of silence, often with the implication of confidentiality, discretion, or focused listening, or to prevent disturbance.

Nuance & Depth
It's an imperative sound, ranging from a gentle reminder to a firm command depending on the tone and context. The prolonged 'ī' sound emphasizes the duration of the requested silence, and the small 'っ' (sokuon) often conveys an abrupt cessation of noise.
Shī- vs Shhh! Hush! Be quiet!: Functions as a direct interjection to halt noise or speaking. It's an iconic sound, much like its English counterpart, universally understood for 'be quiet'. The sound itself imitates the 'sh' sound made to quiet someone.
Imagine This Situation
Example Sentences & Audio
(Shhh! (しーっ, asking for silence) This is a library.)
(The teacher is talking, so be quiet (しーっ, requesting silence).)
(Shhh! (しーっ, indicating secrecy) Someone might hear us.)
Summary
👉 'しーっ' is a direct, onomatopoeic interjection commanding or requesting immediate silence, often with a finger-to-lips gesture. It can also imply a need to maintain quiet for reasons of discretion, listening, or to avoid disturbance.