An isogram is a word in which no letter is repeated:

• METALWORKINGS
• LEXICOGRAPHY
• MALNOURISHED
• THUNDERCLAPS
• UNFORGIVABLE
• AMBIDEXTROUSLY
• UNCOPYRIGHTABLE

Theoretically the limit is 26 letters, but that’s an Everest that no one has scaled. Dmitri Borgmann has conquered some lesser peaks with THUMBSCREW-JAPINGLY (18 letters, “as if mocking a thumbscrew”) and PUBVEXINGFJORD-SCHMALTZY (23 letters, “as if in the manner of the extreme sentimentalism generated in some individuals by the sight of a majestic fjord, which sentimentalism is annoying to the clientele of an English inn”). Maybe what we lack is imagination.