sh definitions

Merriam Webster's

I. interjection Date: 1847 — used often in prolonged or rapidly repeated form to urge or command silence or less noise II. abbreviation share

Oxford Reference Dictionary

int. calling for silence. Etymology: var. of HUSH

Collin's Cobuild Dictionary

also shh You can say 'Sh!' to tell someone to be quiet. (INFORMAL, SPOKEN) CONVENTION