The radio nebula W 50 harbours the relativistic binary system SS 433, which is a source of the powerful wind and jets. The origin of W 50 is wrapped in the interplay of the wind, supernova remnant and jets. The evolution of the jets on the scales of the nebula is a Rosetta stone for its origin.To disentangle the roles of these components, we study physical conditions of the jets propagation inside W 50, and determine deceleration of the jets.The morphology and parameters of the interior of W 50 are analyzed using the available observations of the eastern X-ray lobe, which traces the jet.In order to estimate deceleration of this jet, we devised a simplistic model of the viscous interaction, via turbulence, of a jet with the ambient medium, which would fit mass entrainment from the ambient medium into the jets of the radio galaxy 3C 31, the well studied case of continuously decelerating jets.X-ray observations suggest that the eastern jet persists through W 50 as hollow one, and is recollimated to the opening ∼ 30• . From the thermal emission of the eastern X-ray lobe, we determine a pressure of P ∼ 3 · 10 −11 erg/cm 3 inside W 50. In the frame of a theory of the dynamics of radiative supernova remnants and stellar wind bubbles, this pressure in combination with other known parameters restricts W 50's origin to a supernova happened ∼ 100 000 yr ago. Also, this pressure in our entrainment model gives a deceleration of the jet by ∼ 60% in the bounds of W 50's spherical component, of radius ∼ 40 pc. In this case, the age of the jet should be 27 000 yr so as to satisfy the sphericity of W 50.The entrainment model comes to the viscous stress in a jet of a form σ = αP, where the viscosity parameter α is predefined by the model.