sex

Sex is indeclinable, and so is every Latin cardinal from quattuor to centum. Only unus, duo, tres and the hundreds above ducenti take endings, which is one of the few pieces of unearned generosity in the language. Around sex sit the ordinal sextus, the distributive seni ("six each"), and the numeral adverb sexies ("six times").

The obvious trap is the English word, which is not related: that comes from sexus, "sex, gender", probably from seco, "to cut, divide". Sex the number gives Spanish seis, French six, Italian sei, and in English a whole shelf of borrowings - sextant, sextet, sexagenarian, semester (sex menses, six months) and siesta, which is the sexta hora, the sixth hour after dawn, when sensible people stopped work.