interjection
shucks
SHUKS/ʃʌks/
Expresses mild dismay or modesty.
Shucks, I'm glad you like the craft I made.

Etymology
A shuck is a husk or shell, figuratively something worthless. The interjection turns that notion of little value into a mild dismissal, allowing speakers to minimize either disappointment or their own importance.
In context
He's like, "Oh, shucks. Forgot my disguise." It's like he's talking to them.