plural noun. A gathering of former cheerleaders, often from New Jersey. Their common bond is that they've all had elective plastic surgery to make up for "God's mistakes."
This term is used in a way similar to "a gaggle of geese," "a herd of deer (heard of deer? of course i've heard of deer)," or "a murder of crows." It should never be used in the context of "A Flock of Seagulls," unless the stacies still have big hair.
stacy 1: oh, hi, stacy!! love your hair!! how's your husband, you know, the guy i used to sleep with?
stacy 2: well hello, dear!! he's just fine, thank you. his rash finally cleared up.
stacy 3: hey girls, i can still fit in my old uniform!!
stacy 4: well, considering you were a size 42 in high school, that's no accomplishment.
stacy 5: now that's enough catty talk from you sluts...i mean, you ladies.
don the waiter: oh hell, another group of stacies. lousy tippers and louder than giant stadium.
ted the bartender: don't sweat it man. besides, stacies are good for a quick one out in the parking lot. all you have to do is tell them how young and gorgeous they look.