\documentclass{article}
\usepackage{amsmath,graphicx}
\DeclareRobustCommand{\rneg}{%
{%
\nonscript\mspace{1mu}
\text{\rotatebox[origin=c]{-90}{\reflectbox{$\neg$}}}
\nonscript\mspace{1mu}\mspace{1mu}
}%
}
\begin{document}
$a\rneg b_{a\rneg b}$
\end{document}