An onomatopeic word for an involuntary sigh of
disappointment.
Particularly if
disappointed in oneself.
"When he realised he might lose the high-stakes Words with Friends game, he accidentally let out a thpbt, right onto a
double word
square."
"Hey man did you hear Todd thpbtting when he opened the exam
paper? He thought he didn't need to learn the conjunctive, what a tmook"