'stewardesses' is not in /usr/dict/words, though. neither are 'aftereffects', 'reverberated', 'reverberates', 'desegregated', or 'desegregates', all of which are also 12 letters, same as 'stewardesses'.

if you really want to push it, you could include 'desegregaters', which is 13 letters (although, the real word may be spelled 'desegregators', in which case it isn't left-hand-only). i was unable to find either 'desegregator' or 'desegregater' with Google, but i would imagine many English speakers would find it a grammatical word if someone used it in a conversation. anyone have the Oxford handy?