In Dave Silverman's list of abstemious words in the February 1972 Kickshaws, the most abstemious was CRWTH (pronounced 'crooth'). For those of you who don't know, an abstemious word is one without vowels.
"Creating Consonant Strings,"
Word Ways: Vol. 15
, Article 17.
Available at: https://digitalcommons.butler.edu/wordways/vol15/iss1/17