Smiley with zipper instead of a mouth is keeping a secret for itself. Lips are closed or chat partner's lips should remain closed. Can also mean silence because you cannot find the right words.
The zipper-mouth face emoji was introduced in 2015 and released as part of the Unicode 11.0 standard. This emoji can be found under the smileys and emotions
category. Because the zipper-mouth face emoji has been around for quite some time, it should display correctly in the majority of operating systems.