This expression refers to messy data from variations in spelling or input—Chihuahua is easy to misspell. The quality of your data matters—errors can creep in anywhere, particularly when people enter data. Garbage in, garbage out.
Capitals, spaces, misspellings, hyphens, numbers stored as text, numbers entered as letters (I, O), accents, straight/curly apostrophes, dates out of order, languages, dialects, abbreviations, and more are all routes for misleading your analysis.
Spend time with your data, or maybe think of using QCsolver!
The name The chihuahua syndrome is from Edward Tufte, and the image is from Sketplanations.