The word 'sshh' is a purely onomatopoeic formation, representing the sound made to command silence. It is an elaboration of the simpler interjection 'sh', which has been used in English since the 19th century to demand quiet. The extended spelling 'sshh' visually mimics the act of prolonging the hushing sound for emphasis. As a simplex word, its meaning is derived directly from its phonetic imitation of the act of hushing, rather than from the combination of classical morphemes.

This is an onomatopoeic interjection formed by the reduplication and extension of the sound 'sh', which is a common hushing sound in English.