Plagiarism is the reuse of other's materials without proper credit and acknowledgement 
Credible sources are vaguely defined, but it can be attributed to verifiable facts in the literature 
I want to ask that, in a situation, when an academician found an "inspiring" idea (not necessarily well-defined) on the non-academic part of the Internet, such as some blogs, or discussion on the social networks, a Youtube video, or some crude Python scripts, etc, and work out on their own to shape up an explicit scientific study or, for example, write up a sophisticated numerical package, then two there are two things here: (1) the source does not provide a scientific study, nor in an academic form, thus it cannot be considered as "credible source", so usually it cannot be cited, (2) but it provides a crude idea that one formulates a scientific problem based on it, so not citing it should be considered as plagiarism.
What is the way to deal with it? Whether to cite or not to cite? For a casual discussion on the Internet to inspire research might be rare in reality, at least it is me who hasn't seen a case, but what if this situation came into reality?