The characters combine to paint a vivid picture: 野心 (yěxīn) means “wild heart” or “wild ambition.” 勃勃 (bóbó) describes this ambition as vigorously and visibly bursting out. So, the full idiom means “brimming with a wild, untamed ambition.”
In many Western cultures, particularly in the United States, being “ambitious” is a highly praised virtue. It's associated with the American Dream, drive, and the pursuit of success. A job candidate proudly stating “I'm very ambitious” is seen as a positive. In Chinese culture, the concept is more nuanced. While modern China certainly values success, traditional Confucian values emphasize humility, collective harmony, and fulfilling one's role within a group. Overt, individualistic ambition could be seen as a threat to this harmony. Therefore, describing someone as 野心勃勃 (yěxīn bóbó) is not a straightforward compliment. It's a powerful observation that often serves as a warning. It implies that the person's ambition is for themselves, potentially at the expense of the group. They might be a “lone wolf,” untrustworthy, or someone who will “step on others to get to the top.” While the term can be used neutrally for a determined entrepreneur, it almost always carries this shadow of caution.
The connotation of 野心勃勃 heavily depends on the context and the speaker's intent.