Mammoth Uncensored Word List:
34 - 12 letter words starting with at

atheological atheophobics atheoretical athermancies atheromatous athletically athleticisms athrocytoses athrocytosis athwartships atmospherics atomisations atomizations atriodigital attaintments attemperment attendancies attendements attenuations attestations attitudinise attitudinize attorneydoms attorneyisms attorneyship attractances attractingly attractively attributable attributions attributives atychiphobes atychiphobia atychiphobic