Note: The word crud in the sense "curd" is a regional and dialect variant in the British Isles (see note at curd entry 1). It has also been sparsely recorded in the U.S., usually as plural cruds in reference to cottage cheese, alongside cruddle for curdle and cruddy as either a noun ("curds") or an adjective "coagulating into curds"; the Dictionary of American Regional English, vol. 1 (Cambridge, MA: 1985) labels crud in the dairy sense as "obsolescent." The word, however, displays a number of apparently derived informal or slang senses in American English, as "deposit of filth," "bodily disorder" and "contemptible person"; none of these appear to be attested before the 1930's. In part these senses may be back-formations from the adjective cruddy. This word in a derogatory sense is attested much earlier than parallel senses of crud, in The Molly Maguires and the Detectives (New York, 1877), one of a series of books with Allan pinkerton's name on the title page that purport to be memoirs of detective work and were almost certainly ghostwritten. In the text cruddy appears three times, associated with or in dialogue by Irishmen. It is difficult to judge how much exposure the unknown author had to Irish speech; in any case cruddy in a derogatory sense does not appear to be attested in Hiberno-English.

Note: Compare also Norwegian regional krodde (masculine) "dregs," (feminine) "boiled cheese." The hypothesis of an iterative derivative with an outcome of Kluge's Law is from G. Kroonen, Etymological Dictionary of Proto-Germanic (Brill, 2013). The Oxford English Dictionary, third edition (2019), along with earlier dictionaries, treats the Middle English verb as derivative of the noun crud, curd (see curd entry 1), though if Kroonen is correct, the noun must be secondary. The Oxford editors regard the noun as either descending from an unattested Old English nominal derivative of *krūdan- (with unexplained u as a stem vowel) or as a borrowing from Scandinavian (based on the Norwegian evidence). Kroonen regards the sense "curds" as possibly secondary, given the sense "dregs" of Norwegian krodde, which would rule out an oft suggested relationship between crud/curd and Middle Irish gruth "curds, cheese."