From the corpus of about one million words in the English language, there are about a dozen words (mostly rare or obsolete) that vie for the longest one-syllable (monosyllable) words with 11 or 10 letters. Containing 9 letters are 10 more common words that curiously when read out loud form a rather challenging onomatopoeic tongue-twister.
11-letter monosyllable words:
squirrelled (11 letters) or squirreled (10 letters): To hide in a safe place.
broughammed: To travel by a four-wheeled horse drawn carriage (known as a brougham); Rhymes with “zoomed.”
10-letter monosyllable words:
schmaltzed: Evoked a sentimental atmosphere
strengthed: To strengthen (obsolete)
thrutched: Pushed or squeezed into place
9-letter monosyllable words:
Read related posts: What is the Longest Word in English?
What is the Longest Acronym?
What is the Most Misspelled Word in English?
Words with Letters in Alphabetical Order
Difficult Tongue Twisters
What Rhymes with Orange?
For further reading: Crazy English by Richard Lederer, Pocket Books (1989)