The image of a snake hidden in your bosom, a place of supposed safety and comfort, symbolizes someone who seems harmless but is actually dangerous. The idiom suggests that you are blindly trusting someone who will ultimately take advantage of your kindness and loyalty.
Here are some examples of how it can be used:
* "He's been nothing but kind to her, but she's clearly using him for his money. It's like he's nurturing a snake in his bosom."
* "I'm starting to think I've been nurturing a snake in my bosom by trusting him with my business secrets."
The idiom implies that you're being naive or foolish in your trust, and that you're likely to suffer as a result.