To be opportunistic is to quickly take advantage of a situation, usually in a way that's just plain wrong.
You know how an opportunity is a chance to do something? When you're opportunistic, you take advantage of that chance, usually immediately. Most of the time, this word is not neutral: people described as opportunistic are also considered unethical, like a business taking advantage of employees or customers in an opportunistic way. If someone dropped their wallet and someone else found it and kept it, that would be opportunistic. Being opportunistic is shady and self-serving.