its an antonym
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.
In a religious context, it could be earnest, pious, or devout. With regard to "keeping faith", it could be loyal, steadfast, or resolute.
that way
The opposite could be insincere, or (for enthusiastic) unenthusiastic or unambitious.
He was an earnest man. He performed his task in earnest.
earnest
No, "earnest" does not have a prefix in it.
Matthew Earnest's birth name is Matthew Walter Earnest.
Earnest Rhone's birth name is Earnest Calvin Rhone.
Earnest Byner's birth name is Earnest Alexander Byner.
The look on his face was so earnest that she believed him. His earnest devotion to her was a source of great comfort.
There are 2 syllables in earnest.
Benjamin Earnest is 6'.
The shopkeeper was very earnest to his customers.
Earnest Gray was born in 1957.