r1 is an unpredictable/irregular affix for word/concept r2, with form/properties r3, in language r4.
In Lojban, cunrafsi are rafsi (bound combining forms) that derive unpredictably from gismu (root predicate-words). They are morphemes of the forms CCV, CVC, CVV, and CV'V (where C is a consonant, V is a vowel, and ' is the special sound /h/). Rafsi of these forms must be memorized alongside their source gismu, since they are assigned arbitrarily (within some constraints). By contrast, rafsi of the forms CCVC and CVCC are always predictable, and match up one-for-one with gismu; these may be called jvarafsi ('rule-rafsi'). Note also: cunrafsi are not 'irregular' in the sense of being assigned informally in a non-standard manner; they are a part of standard, official Lojban.