report in sick
To inform one's employer that one will be absent due to illness (real or feigned). That virus has totally decimated my staff—only two people are in the office today because the rest reported in sick! If you really want to go to the beach tomorrow, just report in sick.
report in sick
to call one's office to say that one will not come to work because one is sick; to show up for work ill. (See also call in sick. Normally, one must be present to report in.) I don't feel well today. I will report in sick. The phone was busy, so I reported in sick by e-mail.