Lewis & Short

immĭsĕrĭcors (inm-), ordis, adj. [inmisericors], pitiless, merciless (very rare, but class.): ipsum immisericordem, superbum fuisse, Cic. Inv. 2, 36, 108: judex immisericors atque inexorabilis contra improbos, Gell. 14, 4, 3.
Of things: flucti (i. e. fluctus) inmisericordes jacere, Att. ap. Non. 488, 12 (Fragm. Trag. v. 33 Rib.); Vulg. Jer. 50, 42.
* Adv.: immĭsĕrĭcordĭter, unmercifully: factum a vobis duriter immisericorditerque, Ter. Ad. 4, 4, 28.