2767: Rigor and Rigid Jul 18, 2024

Traditionally, nouns ending -or, as part of the word itself and not with an added suffix, took the ending -id as an adjective, like horror–horrid, splendor–splendid, and squalor–squalid; others, meanwhile, lost this and took the adjectival suffix -ity, like frigid-frigity, and morbid-morbidity. An even smaller set of these words have both forms, though usually this is because the nominal and adjectival forms have divergent meanings, like humor–humid–humidity, stupor–stupid–stupidity, or valor–valid–validity, all of whose nominal forms now take the -ous ending.

Perhaps the most interesting example of this is rigor–rigid–rigidity, because while this also completely diverged in terms of the meanings of the original noun and adjective pair, the original meaning of ‘rigor’ not meaning ‘thoroughness’ but rather ‘stiffness’ is maintained in general in the medical field, and in particular in the phrase ‘rigor mortis’ (literally ‘rigidity of death’). Of course, this phrase is from Latin and does not carry the modified meaning that ‘rigor’ has had in Modern English.