force of habit; momentum of habit
Describes continuing an action or state simply because one has been doing it, without conscious thought or fresh intention. Often used in expressions like 惰性で続ける.
毎日同じ道を通るのは、もはや惰性だ。
Taking the same route every day is just force of habit by now.
I've been sticking with my current job for years now, purely out of habit.
惰性で仕事を続けているだけの状態から抜け出したい。
I want to break out of the state of just continuing work out of sheer momentum.
Rare physics term for inertia; the standard modern term is 慣性 (かんせい).
See also: 慣性
古典力学の文献では「惰性」が慣性の意味で使われることがある。
In classical mechanics texts, 惰性 is sometimes used to mean inertia.
Repeating the same daily routine, I ended up falling into a passive rut.
I want to break out of my rut by starting a new hobby.
Without any goals, I'm just drifting through my days out of habit.
慣性 is the standard modern physics term for inertia, whereas 惰性 is a rare, old-fashioned synonym in that technical sense.
マンネリ describes a stale, repetitive rut (especially in work or relationships), while 惰性 focuses on the lack of conscious will behind continuing something.
From 惰 (だ, laziness, inertia) + 性 (せい, nature, quality). The kanji 惰 suggests a passive, unmotivated continuation, which fits both the 'force of habit' and the rare physics sense.