'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?