mirth ‧ less /ˈmɜːθləs $ ˈmɜːrθ-/ BrE AmE adjective
literary mirthless laughter or a mirthless smile does not seem to be caused by real amusement or happiness:
‘Now it’s your turn,’ he said with a mirthless grin.
—mirthlessly adverb :
He smiled mirthlessly.