The word 'umpteenth' is a playful, informal coinage from the early 20th century. It derives from 'umpty', a humorous slang term for an indefinite number, which itself was patterned after military telegraphic code words for the dash in Morse code (like 'iddy-umpty'). The suffix '-teenth' is borrowed directly from the series of ordinal numbers (e.g., thirteenth, fourteenth), giving the word its sense of being one in a long, repetitive sequence. Thus, 'umpteenth' creatively combines a nonsense root with a familiar grammatical ending to express a concept of vague, often exasperating, multiplicity.

The word is a simplex formation. The element 'ump-' is an arbitrary, fanciful extension of the earlier slang word 'umpty' (meaning an indefinite number), combined with the ordinal suffix '-teenth', modeled on numbers like 'thirteenth', 'fourteenth'.