Is there any basis to the claims he was a plagiarist?

No. Notice how these claims never come from people who actually understand the "plagiarized" material (physicists), and the people he supposedly plagiarized from were perfectly willing to give him credit. It's just a /pol/ meme because he's the only Jewish scientist they know.

>>1195371
What is he supposed to have stolen? Lorentz Transformation?

>>1195774
Poincare's ideas as well as Lobatchevski's
>inb4 on giant's shoulders

>>1195780
Well poincare groups are rather fundamental >shoulders of giants.

>>1195371
Physicist here. He didn't plagiarize or steal, he just unfairly gets credit for everything when relativity was built from the contributions of many scientists.

It's pretty clear that he didn't really have much of an idea of what he was doing. In his 1904 paper, the one that people think "invented special relativity," he writes c+v