A combination of the word "Skinny" and "Inches".
The word itself means "Skinny inches". Meaning someone who is skinny, and has long hair.
Skinny being a word derived from stan twitter, and inches pointing to a person with long hair.
Word-play on "Girl". Coined by Pentatonix member Mitch Grassi. Mitch Grassi also runs a YouTube channel with fellow Pentatonix member Scott Hoying, and has said this word in one of their videos.