Heini means "fit" in Welsh. Welsh Heini's are fit, as in athletic and fit generally. They are bright and bouncy too, although given to drinking too much on occasion, and walking oddly when drunk and in heels. In essence, a floozy in a onesie.
Welsh Guy 1: W yli ma'r hogan yna yn heini (Eng: Ooo that girl's fit)
Welsh Guy 2: Ond mae hi'n cerdded fel y Grinch (Eng: Ye but she's walking like the Grinch)
A very cool girl who is an amazing friend. Heinis typically take in the weird introverts and collect them into their friend groups, they are kind and smart and everyone likes them.