Meaning of the word "in earnest" in English

What does "in earnest" mean in English? Explore the meaning, pronunciation, and specific usage of this word with Lingoland.

in earnest

US /ɪn ˈɜːrnɪst/
UK /ɪn ˈɜːnɪst/

Phrase

seriously and sincerely

Example:
He began to work in earnest on his new project.
The negotiations only started in earnest after the initial discussions.