GUESS THIS!!!

There are actually a couple: rhythm syzygy (the alignment of three celestial bodies) crypts lymphs nymphs synchs

rhythm

The words contain the so-called semivowel y and none of the true vowels.But then by defn a vowel is one of a class of speech sounds in the articulation of which the oral part of the breath channel is not blocked and is not constricted enough to cause audible friction;broadly : the one most prominent sound in a syllable The other defn with aeiou & y as semivowel .....is for kids/ people that cant/donot intend to think.:) So prolly there is no 6 letter english word completely vowelless

rythmmmm