In March 2018, a question in the knockout competitions (Cup and Plate) of Stockport Quiz League asked: "Which is the longest word in the English dictionary which does not contain a vowel?"
The answer given on the sheet was 'rhythm'. The questions guru pointed out that 'rhythms' is a longer word, and has no vowels; but I don't think you'll find that word – or any other plural that's formed simply by adding an 's' to the singular – in many dictionaries.
Neither the setter not the guru seems to have thought of 'syzygy', which is when three or more celestial bodies in a gravitational system move into a roughly straight–line configuration. An example of syzygy is when the Moon moves into a position directly between the Sun and the Earth, which causes a solar eclipse.
There may be other words that fit the bill – possibly containing more than six letters. So I've left the 'question' open; if you know of any others, please let me know!
© Haydn Thompson 2020