When a hot girl has skinny legs, forming a gap between her thighs. This would hypothetically allow one to "cup" her vagina with a hand while she is standing up. In its more moderate form, it results in an upside down triangle shaped gap, which is also quite hot.

*Not to be confused with the optical illusion created by larger girls squeezing into tight jeans.
"Wow, look at the cupability on that chick." or; "Damn, that girl's got negative cuppa."
by DJEasydick January 24, 2010
Get the cupability mug.