in a serious manner
"talking earnestly with his son"
"she started studying snakes in earnest"
"a play dealing seriously with the question of divorce"
In a serious or literal manner.
"He asked me to take his offer seriously, not as just another joke he tells at parties."
In plain English: Seriously means doing something with a lot of focus and real importance instead of just joking around.
"She was seriously injured in the car accident."
Usage: Use seriously to describe doing something with full attention and without joking, such as taking an exam or considering an offer. It often contrasts with casual remarks but does not necessarily imply the subject matter is grave in tone.
The word entered Middle English from the French sérieusement, meaning "earnestly." It was formed by adding the suffix -ly to the adjective serious.