Wǒ zhēn shì ge shǎmào, jìngrán wàng le dài yàoshi.
I'm such an idiot — I actually forgot to bring the keys.
Tips
register
傻帽 is northern Chinese slang, especially Beijing-flavored. Lighter than 笨蛋 in feeling — closer to 'goofball' or 'dummy'. Use among friends or self-deprecatingly; can sting if directed at a stranger.
memory
傻 (silly) + 帽 (hat) — picture someone wearing the proverbial dunce cap. The 帽 is just a colloquial suffix here (compare 二百五的帽子 in slang).
