I'm trying to get this expression to display as typed instead of evaluating the 1/1.121.

SetAttributes[TraditionalForm, NHoldAll];


After reading the documentation I would have thought NHoldAll would have stopped this but it doesn't seem to.

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The correct attribute to set would be HoldAll, but modifying built-in functions like this (setting attributes that affect evaluation) is very likely to break things.
Instead, use HoldForm:
TraditionalForm@HoldForm[(17.517*CuS^2 - 12.081*CuS + 54.875)/(1.121)]