'Most Useless / Pathetic Person Ever Trained'
Common usage has seen the term become a day-to-day reference for someone who is particularly stupid, clumsy or lacking common in sense and logic.
Origins: Unknown - however most evidence points to the film industry (particulary gaffers & grips) rather than the US Military who is usually given the credit for such acronyms.
Tom? - Hell no, He's a total muppet."