Major dictionaries like the Oxford English Dictionary document standard spellings (e.g., “sexy”) and common slang forms when widespread; highly nonstandard stylizations (repeated letters for emphasis) are generally treated as informal orthographic variation and not separate headwords. If you need an authoritative definition for formal use, cite “sexy” rather than “sexxxxyyyy.”

The extra 'x' and 'y' characters act like a verbal exclamation point.