If you've ever spun in circles until you fell to the ground laughing, you know how it feels to be giddy. This adjective can mean dizzy, elated, or — as in the spinning around example — a lightheaded, lighthearted combination of the two.
The hackneyed phrase "giddy as a schoolgirl" calls forth the image of a kid giggling with her friends over some adolescent foolishness. Giddy has been used to describe someone incapable of serious thought or easily excited as far back as the 16th century. Given that, in modern usage, giddy describes someone silly and frivolous, it's interesting to know that the Old English source for this word has a slightly darker tinge: gidig means "insane" or "god-possessed."