May either be a directcompound ofN(the initial letter ofnigger, or anellipsis ofn-word) +tower, or ablend ofn-word +tower. Alternatively still, possibly asyncope ofN-word tower.
Unliken-word, this term may resist being spelled with alowercaseN to a greater degree, since the format of such towers often explicitly features image macros of capitalNs.
(Thisetymology is missing or incomplete. Please add to it, or discuss it at theEtymology scriptorium.)