an abbreviated portmanteau of Internal Monologue, as In(ternal)Mo(nologue)
Used in a literary sense to repsresent the thoughts of the narrator in another character, often exaggerating and lampooning the narrator in the process.
My lungs were vacuous as she approached me. She pushed her hair back and smiled from only the corners of her mouth.
"Hello," I managed to exhale.
"Nice legs," added the InMo, swirling his scotch