Someone who is 'straightish' is someone that is straight as far as the normal beholder, but has sex with the same gender behind closed doorson a regular basis. One can seem (and be therefore deemed) straightish, or it may have been proven.
"His outfit is perfect! Is he gay?"
"I mean... he's straightish."
Comes from the words straight and imposter. Straight + Im(poster)
When a person is a straightposter. Then they are acting gay while being straight. Not in the way of being gay like being flamboyant and that. But for example, sleeping with the same gender or touching the same gender sensually.
But don't bully someone with this. A straight person can have sex with the same gender while being straight.
A: "I heard that Jimmy slept with Tod last night. Even Jimmy himself agrees to that. But he says that he's straight"
B: "He's such a straightposter"