間抜け also means 'fool' or 'blockhead', but emphasizes a lack of sharpness or awareness, whereas 頓馬 often implies clumsiness or slow-wittedness.
Etymology
The exact derivation is uncertain. The kanji 頓 (sudden, dull) and 馬 (horse) are ateji, and the word may have originated as a slang term in the Edo period.
