1. when visible knees (or sometimes legs) are of a distinctly different color to the rest of the body. usually from when tanning of the legs is done separately from the upper body.
2. to describe someone with jaffa knees (said quickly as one word, like "Japanese")
The use of 'Jaffa' is borrowed from the Jaffa chocolates - chocolate confectionery with an orange candy shell - describing the either particularly orange tan, or even dark brown (almost chocolate) color of the said knees
Fred: Hey, did you see that girl? Her knees were like, brown!
Ulric: She's got Jaffa knees.