Exactly what it says it is. No effects when worn or carried (fortunately). They can be found in abundance in Gehennom.

Sacrificing a cheap plastic imitation of the Amulet of Yendor at an altar screws you over big time, with huge penalties on luck and quite probably invoking severe divine retribution. From the NetHack source code (pray.c), in the block dealing with offering items for sacrifice on an altar:

if (otmp->otyp == FAKE_AMULET_OF_YENDOR) {
            if (flags.soundok)
                You_hear("a nearby thunderclap.");
            if (!otmp->known) {
                You("realize you have made a %s.",
                    Hallucination ? "boo-boo" : "mistake");
                otmp->known = TRUE;
                change_luck(-1);
                return 1;
            } else {
                /* don't you dare try to fool the gods */
                change_luck(-3);
                adjalign(-1);
                u.ugangr += 3;
                value = -3;
            }
    } /* fake Amulet */

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