half out of interest; mostly out of curiosity
Used when someone does something motivated partly by curiosity, often implying it's not a serious or committed reason. Commonly used in the adverbial form 興味半分で.
I started reading the book half out of curiosity, but ended up finishing it.
He tinkered with the machine half out of curiosity.
Compound of 興味 (interest) and 半分 (half). The expression is transparently derived from its components.