What does «inmemorable» mean in Spanish?

So old, there is no memory of when it began or its origin; immemorial.

Examples of use in Spanish: "esto se hace así desde tiempos inmemorables".

"la ciudad es de inmemorable fundación".

"una costumbre que data de tiempo inmemorable".
That it cannot, should not or is not worthy of being remembered; as opposed to memorable.

Examples of use in Spanish: "es un texto tan malo que es inmemorable".