When you undress from the line it's like you're peeling something off the line, going off in another direction, and then causing a digression in whatever the line was about. It's like when you're talking and you digress. Think of your speech as a line. Then, undress a part of it and let it fall away. That is your digression.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.