"A collection of insults 3" joke

Low-bandwidth as an information source.

Lugnuts rattling in the hubcaps.

Made a career out of mid-life crisis.

Mainspring's wound too tight.

Makes a black hole look bright.

Makes predictions that make weathermen/economists look good.

Memorized every Dr. Seuss story written.

Mental software is Version 1. 0 / still in beta test.

Mentally qualified for handicapped parking.

Metronome needs oil.

Might still be a virgin except for what nature did to her mind.

Mind like a steel sieve.

Mind like a steel trap -- everything gets mangled / full of mice / nothing in, nothing out / rusted shut.

Missing a few buttons on his remote control.

Missing a few catalog cards / gears / marbles.