Japanaese onomatopoeia indicating the 'sound' of silence.
Use this for all variants of *shiin*, including *shin*, *shiiiiiiiiiiin*, *shiiin-to*, etc.
Chinese also has a similar onomatopoeia as "静... " (jin...), with a similar sound. This is sometimes also used to translate the Japanese しーん. If you feel it is appropriate, you may tag such examples also with shiin_(sfx).
