Wichser is a common german insult with not much meaningful impact behind itself (
see the background information later).
It is only used for male targets, because "wichsen" or "wixxen" is a synonym for (ejaculation) from masturbation. Sometimes is it used ironically from friends.
Wichse = cum
Wichser = (bad) person
Wichsen = masturbation with ejaculation
The background:
Masturbation was considered a sin by religious people. So a "Wichser" was a sinner, a bad person.
In modern days the background can be another:
If you need to
masturbate, you have no girlfriend because you are a
loser or
ugly/ unlikeable.