late 14c., from a common PIE imitative *mut- "to grunt, mutter" (cf. O.N. muðla "to murmur," L. muttire "to mutter"), with freq. suffix -er. Related: Muttered; muttering.
To quietly enter a command not meant for the ears, eyes, or fingers of ordinary mortals. Often used in "mutter an incantation". See also wizard. [Jargon File] (1995-02-06)