Source has Voiceroid tagged, but I'm not familiar with it so someone should double check if this image references it in some way or if it's just a mistag at source.

This was tagged with ｺｯｼｮﾘ "kosshori", which is included in the Other Names for VOICEROID. It is a cutesy way of spelling "kossori", meaning "stealthily/secretly". So kinda like "secwetly"

According to Pixipedia, this tag is associated with VOICEROID because it is used to label R-18 content. The official VOICEROID accounts were generally accepting of R-18 fanart of their characters. However, the official Touhoku Zunko Twitter account was supposed to be for all ages. This account frequently retweeted fanart, and once accidentally retweeted R-18 fanart of Touhoku Kiritan. After that, they decided to make a tag to filter out R-18 posts, and thus ｺｯｼｮﾘ "secwetly" was created.

As for why this post in particular was tagged, I guess it's either referencing some R-18 VOICEROID art, or perhaps just her general pst-pst whispering stance that... still has a mild sexual innuendo whatwith the pointing and laughing.