also: でこすけ · デコスケ
noun
big forehead; fivehead; asshole
Derogatory Kantō dialect term for a person with a large or prominent forehead. The 'asshole' gloss reflects a strong insult usage, not a literal anatomical meaning.
あのデコ助、また自慢してるよ。
That fivehead is bragging again.
デコスケって呼ばれて怒った。
He got mad when they called him 'big forehead'.
From でこ (forehead, dialectal/abbreviated form of でこっぱち) + 助 (suke, a common suffix in male names or derogatory nicknames). The exact origin is uncertain, but it likely arose as a teasing or insulting nickname in the Kantō region.