meaning from the freedictionary.com

(right) off the top of one’s head

without giving it too much thought or without precise knowledge

off the top of your head  (informal)

if you say something off the top of your head, you say it without thinking about it for very long or looking at something that has been written about

Trivia: koekikoe is my little sister’s cake shop. She makes cupcakes based on orders 😀