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We present a comprehensive spectro-polarimetric study of persistent Blackhole X-ray binary 4U $1957\!+\!115$ with {\it IXPE} and {\it NICER}observations. The source is observed in disk dominated thermal state with disktemperature, kT$_{in}\approx1.4$ keV. The emission during thermal state fromthe source is found to be moderately polarized and \textit{IXPE} measures adegree of polarization (PD) $= 1.95\pm0.37\%$ ($>4.79\sigma$) along with apolarization angle (PA) $= -42.78^{\circ}\pm5.41^{\circ}$ in the energy rangeof $2-8$ keV. PD is found to be an increasing function of energy, whereas PAindicates switching within the energy range which could be due to highinclination and the returning radiation within the system. Simultaneous energyspectra ($0.6-10$ keV) from {\it NICER} are modelled to study the spectralproperties. Furthermore, the spin parameter of the black hole is estimated withspectro-polarimetric data as a$_{\ast}=0.988\pm0.001\,(1\sigma)$, which iscorroborated by {\it NICER} observations. Finally, we discuss the implicationsof our findings.

Spectro-polarimetry of 4U 1957+115 with IXPE: Effects of spin and returning radiation on polarised emission of black hole in thermal state