A fibber is a person who tells fibs — small, unimportant lies, often a bit silly: “I didn’t eat the cake” when there is chocolate on your face. We use it with children, and often with a smile. It is informal, and a little old-fashioned. It is kinder than liar. A liar might hurt you on purpose; a fibber is usually just wriggling out of something small. The word names the person, not the act: the verb is to fib. You would not say “he fibbered”; you would say “he fibbed,” or “he’s a fibber.”