expression
with stealthy steps; on tiptoe; stealthily
Describes moving quietly so as not to be heard; often used in narrative or descriptive contexts.
彼は足音を忍ばせて部屋に入った。
He entered the room with stealthy steps.
足音を忍ばせて近づく。
Approach on tiptoe.
こっそり is a general adverb for doing something secretly or stealthily, while 足音を忍ばせて specifically focuses on moving without making footstep sounds.
A set phrase combining 足音 (footsteps) with 忍ばせて, the te-form of 忍ばせる (to conceal, to keep quiet). Literally 'concealing one's footsteps'.