For a male penis to go from a state of softness or relaxation to a state of an uncontrollable erection in the penis.
Mainly caused by a beautiful or attractive female when seen or spotted with the eyes.
A combination of the words: Instantly (Without Delay or Immediately) and Hard (Not easily penetrated).
"Man, I just got instahard when she walked in"
"Lola Luv = Instahard"