1
general::
On duty or at work. For example, Despite his illness he's determined to continue in harness. It Also is put as be back in harness, meaning "to return to duty or work," as in After a long vacation she's finally back in harness. This expression alludes to horses harnessed to perform work. [First half of 1800s] Also see DIE WITH ONE'S BOOTS ON (IN HARNESS).
American Heritage Idioms