\documentclass{article}
\usepackage{bm}
\def\makebm#1${\bm{#1}$}
\renewcommand*\descriptionlabel[1]{%
  \hspace\labelsep
  {\everymath{\makebm}\normalfont\bfseries #1:}%
}
\begin{document}
\begin{description}
\item[$a^{b^c}$] $a^{b^c}$
\end{description}
\end{document}

