Being on the same wavelength is similar to the expression "We're totally on the same page" or "Great minds think alike" It means that you and the other person are similar in thinking at that very precise moment. Often this happens when you voice a thought and someone else says the exact same thing or something similar. It just means you had the same opinion on something about a certain topic, issue or whatever.
"That skirt is to die for. It would go so well with the heels I bought last Saturday"
One of those annoying creeps who pretends he is into all the same things as a girl to try and get into her dark place. They possess some sort on stealth powers as the girls can never see that they are being tricked. Dumbasses.
Fontaine: That sod Mark is wavelengthing my girlfriend when I'm at work, trying to get her to play a rusty trombone on him.That fucking wavelengther bastard chutney ferret.
Garfield: Meh, she's a porker anyway, let him have her.
The term describing a subconscious act of sharing memes at unconventional times of day/night you and a friend experience at pure coincidence.
Croeh I got up to get a glass of water at 4 A. M., opened my chat window to send a meme. He was typing, just sent me one before I could!
Lupy: You guys are on the Dank Wavelength, bro!