The opposite could be insincere, or (for enthusiastic) unenthusiastic or unambitious.
He was an earnest man. He performed his task in earnest.
The crowd looked on in earnest. Jack awaited the arrival of the Post man in earnest.
that way
I will tell you the earnest truth. I want to be earnest with you: you're not the one for me. I earnestly want to meet you. Your earnestness is one of your greatest virtues. Not everything is a game somethings are earnest.
unenthusiastic
Serious, earnest.
No, earnest means characterized by a firm and humorless belief in the validity of your opinions.
He was an earnest man. He performed his task in earnest.
earnest
No, "earnest" does not have a prefix in it.
Earnest Rhone's birth name is Earnest Calvin Rhone.
Matthew Earnest's birth name is Matthew Walter Earnest.
Earnest Byner's birth name is Earnest Alexander Byner.
There are 2 syllables in earnest.
Benjamin Earnest is 6'.
The look on his face was so earnest that she believed him. His earnest devotion to her was a source of great comfort.
The shopkeeper was very earnest to his customers.