Always ready for sex. A woman who is always lubricated or a man who is always erect. Can also be said of an animal.
"eveready" is usually considered a compliment, describing someone who is horny, but not necessarily slutty. Sometimes used with a bit of anxiety by someone who can't keep up with a partner's level of sexual desire.
"My new girlfriend is great. She's eveready. I started banging her while she was asleep and she was already wet."
"Have you seen how he always has a bulge in his pants?" "Yeah, he's eveready."
"Wow. Look at your dog's...uh...thing." "Oh yeah, he's eveready."