HoldComplete[expr] shields expr completely from the standard Mathematica evaluation process, preventing even upvalues associated with expr from being used.
HoldComplete has attribute HoldAllComplete, and performs no operations on its arguments.
HoldComplete is removed by ReleaseHold.
HoldComplete can be inserted as a wrapper by such functions as ToExpression and ReplacePart.
HoldComplete is generated by default by MakeExpression.
See The Mathematica Book: Section 2.5.5 and Section A.4.4.
See also: Hold, HoldPattern, HoldForm, Unevaluated, HoldAllComplete, Symbol.