what does "spitless"mean?
“She didn’t just fall.” The sudden certainty of it punched like a fist. “The son of a bitch. Did he push her, trip her, just scare her so she lost her footing? Then he left her there. He left her on the floor.”
“We need to call Vinnie.”
“It can wait till morning. This isn’t going anywhere. I turned the wrong way. To get the wrench. I got mixed up. It’s been years since I’ve been down here, and I went the wrong way. We used to scare ourselves spitless in here when we were kids. It’s the oldest part of the house. Listen.”
When he fell silent, she heard it clearly. The grumble of wave over rock, the moan of wind.

Question

what does "spitless"mean?
“She didn’t just fall.” The sudden certainty of it punched like a fist. “The son of a bitch. Did he push her, trip her, just scare her so she lost her footing? Then he left her there. He left her on the floor.” 
“We need to call Vinnie.” 
“It can wait till morning. This isn’t going anywhere. I turned the wrong way. To get the wrench. I got mixed up. It’s been years since I’ve been down here, and I went the wrong way. We used to scare ourselves spitless in here when we were kids. It’s the oldest part of the house. Listen.” 
When he fell silent, she heard it clearly. The grumble of wave over rock, the moan of wind.

Answer

I have never heard it before, however it is safe to assume the writer means so scared that their mouths became dry

Answer

The phrase is "scare someone spitless". We use "spit" to replace a more vulgar word. It's not a literal phrase.

Answer

I agree with Joseph,but it is an expression the writer has invented I think,as I have also never come across it before