It comes from “mannerism”. It is usually used as “マンネリ化(か)する” meaning “to get stuck in a rut”. It is often used to describe a couple who gets stuck in a relationship rut.
Recently, I feel stuck in a relationship rut with my boyfriend.
日本語の勉強が マンネリ化 して、やる気(き)が出(で)ない
I feel stuck in a rut with studying Japanese and I don’t feel motivated.