noun
quiet weeping; silent crying
Crying quietly so as not to be noticed; often implies suppressed sobs or tears shed in secret.
彼女は布団の中で忍び泣きをしていた。
She was weeping quietly under the futon.
隣の部屋から忍び泣きが聞こえた。
I heard quiet weeping from the next room.
Compound of 忍び (stealth, secrecy) and 泣き (crying). The exact historical derivation is uncertain, but the meaning is transparent from its components.