when someone of the female persuasion catches you off-guard with a surprisingly strong set of mammary glands.
Most commonly displayed on hippie/bohemian-type women (long dresses, long or unkept hair, abundance of scarves, etc) who are not normally associated with large or well-rounded breasts. When you see a woman of this description with a great set of cans, it can catch you off guard - (1) because they are great in size, circumference, and viewing (cleavage) angles, and (2) because hippie-lady types often don't wear bras, showing these goods in just about their natural (naked) state.
"Your friend looked much better in person than in her pictures, like a classic case of hippie tits, since I got the impression she was too much the athletic or demure type to have great ones".
When you interact with an attractive lass, only to realize immediately that along with her obvious good looks, she has the personality of a wet mop. Outside of outright rejection, nothing is more disappointing than approaching a woman to introduce yourself, hit on her, and find out she has the social and conversational skills of an invalid. The 'Betty" is tribute to the character from the 'Archie' comics who shared a similar disposition.
"So I saw this cute girl out and immediately after I introduced myself it was clear she was another Boring Betty - all looks, no substance."