What's the meaning of "triggeronomy?"

I never heard of the word "triggeronomy." What does it mean?

“She’s bright,” Harlan said. “Mrs. McReady at school says Shan’s the brightest pupil she’s taught in her whole career, and that stretches back almost 40 years. She’s good in English, and she’s even better in the maths, which Mrs. McReady says is rare in girls. She can do triggeronomy, Wilf. Did you know that? Mrs. McReady herself can’t do triggeronomy.” No, I hadn’t known, but I knew how to say the word. I felt, however, that this might not be the time to correct my neighbor’s pronunciation. -- Full Dark, No Stars , Stephen King

The word triggeronomy is the father's impression of the word trigonometry. He has fused the first half of the word with an ending that's commonly heard in other scientific contexts, probably from the word astronomy. The father is showing off here about his daughter, but the speaker is mainly noticing the fact that, ironically, the father doesn't know what trigonometry is or how to say it. Triggeronomy is not a real word.