What is katai?

Is a Japanese word for "Hard", especially in the male genitalia. Or if you're watching Hentai, then you know what I mean...

これわかたいお (Kore wa katai o) - This is Hard

katai - meme gif

it means anything and everything, whatever you want it to mean and can be a substitute for any word, even if they are opposite in meaning

OMG that shirt is so katai I love it!! or OMG that shirt is so katai I hate it!