The sad face with the thermometer in the mouth expresses illness. Caught a flu or other illness that is associated with fever. Or is worried about getting sick.
The face with thermometer 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 face with thermometer emoji has been around for quite some time, it should display correctly in the majority of operating systems.