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.
Taking the same route every day is just force of habit by now.
I want to break out of the state of just continuing work out of sheer momentum.
I've been sticking with my current job for years now, purely out of habit.
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.