\documentclass{book}
%\usepackage[english]{babel} % dies if babel is used
\newcommand\gobbleone[1]{}
\newcommand{\idxmark}[1]{{\let\gobbleone\relax\markboth{\sffamily\bfseries#1}{\sffamily\bfseries#1}}#1}
\begin{document}
\idxmark{thing\gobbleone },
\end{document}