Not willing is a negation of the adjective willing. Unwilling is a single-word synonym for not willing. Here's a list of synonyms for unwilling. “He was restless and seemed unwilling to be alone.” “Forcing this therapy on an unwilling patient is a serious ethical breach of professional obligation.”

These words describe not wanting to do something. One of the most common words for this is unwilling. She was unwilling to hand over the money. He was a very unwilling helper. The opposite of unwilling is willing. I'm more than willing to help out if needed.