Tee hee is defined as a sound made to signify a laugh, especially a child's giggle.
(interjection)An example of tee hee is what a six year old girl might say if she is amused by a joke.
See tee-hee in Webster's New World College Dictionary
interjection, noun
Origin: ME: echoic
intransitive verb tee-heed, tee-heeing
See tee-hee in American Heritage Dictionary 4
also te·hee
noun