だせい
force of habit; inertia
Common word Also written 堕性
Noun
惰性だせいに身みを任まかせているがために今いまのような現在げんざいがあるんですね。
I've ended up in my present state from having indulged myself.
高校時代こうこうじだいからつきあっている彼氏かれしはいるものの、つきあいが惰性だせいになっていると感かんじ、不満ふまんをつのらせている。
She has a boyfriend she's been going out with since high school but feels their relationship has become a matter of habit and is increasingly dissatisfied.