Missing plurals

-UM but not -A

Many words ending in -UM are nouns from Latin, and therefore allow the -A form plural; for example, REFERENDUM and REFERENDA. This section lists words which end -UM but do not admit such a plural and (as far as can be determined) the reason why.

Mass nouns: ALLUVIUM, AVIZANDUM, BDELLIUM, BUNKUM (also not Latin: named after Buncombe County in North Carolina), COLLUVIUM, COLOSTRUM, CORIUM, CORUNDUM, DELIRIUM, DENTALIUM, DEUTERIUM, DIDYMIUM, DOMINIUM, ELECTRUM, EPICARDIUM, EPIMYSIUM, FALCIPARUM, GALBANUM, GYPSUM, HOKUM, IMPERIUM, INDECORUM, KUMKUM (also from Sanskrit, not Latin), LABDANUM, LADANUM, LAUDANUM, LINOLEUM, MECONIUM, MYOCARDIUM, MYOMETRIUM, OAKUM, ODIUM, OLEUM, OPPROBRIUM, ORICHALCUM, PASPALUM, PERIMYSIUM, PETROLEUM, PROTIUM, PUERPERIUM, SARGASSUM, SEBUM, SESAMUM and most chemical elements (see below).

Chemical elements: ACTINIUM, ALUMINIUM, AMERICIUM, BARIUM, BERKELIUM, BERYLLIUM, BOHRIUM, CADMIUM, CAESIUM, CALCIUM, CERIUM, CHROMIUM, CURIUM, DUBNIUM, DYSPROSIUM, ERBIUM, EUROPIUM, FERMIUM, FRANCIUM, GADOLINIUM, GALLIUM, GERMANIUM, HAFNIUM, HASSIUM, HELIUM, HOLMIUM, INDIUM, IRIDIUM, LANTHANUM, LAWRENCIUM, LUTETIUM, MEITNERIUM, MOLYBDENUM, NEODYMIUM, NEPTUNIUM, NIOBIUM, NOBELIUM, OSMIUM, PLATINUM, PLUTONIUM, POLONIUM, POTASSIUM, PROMETHIUM, RADIUM, RHENIUM, RHODIUM, RUBIDIUM, RUTHENIUM, SAMARIUM, SCANDIUM, SEABORGIUM, SELENIUM, SODIUM, TANTALUM, TECHNETIUM, TELLURIUM, TERBIUM, THALLIUM, THULIUM, TITANIUM, URANIUM, VANADIUM, YTTERBIUM, YTTRIUM. (Note that some chemical elements do admit plurals, but that's for another article.) Also COLUMBIUM (old term for niobium) and HAHNIUM (rejected name for dubnium and hassium), JOLIOTIUM (rejected name for dubnium), LUTECIUM (alternative name for lutetium).

Adjectives: ANTEBELLUM, ANTEPARTUM, DINKUM (also adverb), GLUM.

Trees and plants (often do not admit Latinate plurals even when the root word is Latin): ALLIUM, ALYSSUM, ANTHURIUM, ARUM, CALADIUM, CAPSICUM, CERASTIUM, COLCHICUM, CYMBIDIUM, DELPHINIUM, DORONICUM, EPIMEDIUM, EPIPHYLLUM, ERYNGIUM, GELSEMIUM (secondary sense is a mass noun), GERANIUM, GEUM, GUAIACUM, HELENIUM, HYPERICUM, LABURNUM, LIGUSTRUM, LYCOPODIUM, NASTURTIUM, ORIGANUM, PSYLLIUM, PYRETHRUM, SEDUM.

Modern Latin (tends not to admit traditional Latin plurals): PHORMIUM, POLYGONUM, POMATUM, RHIZOBIUM,

Noun in English but adjective in Latin: DECORUM (seemly), INFIMUM (lowest), MAGNUM (great), MODICUM (moderate), NATATORIUM (swimmer-related), NOSTRUM (our), PLENUM (full), SANCTUM (holy)

Noun in English but gerundive in Latin: DEDENDUM (thing that can be surrendered), HABENDUM (that is to be had), JEJUNUM (fasting), PENDULUM (hanging down).

Noun from Latin but no Latinate plural for unknown reason: ASYLUM, LOMENTUM, MENTUM, MUSEUM, PEPLUM, PSALTERIUM, QUADRIVIUM

Not Latin-:

• Chinook Jargon: SKOOKUM
• Dzongkha: CHETRUM
• French: HARMONIUM (Latin is harmonia)
• German: DIAZONIUM, MEERSCHAUM
• Greek: EUPHONIUM (given Latin ending)
• Hindi: CHILLUM
• Malay: PANTOUM
• Middle English: OLIBANUM, OPIUM, SCUM (ultimately Germanic)
• Old English: PLUM
• Old French: CARDAMUM, GUM (ultimately from Egyptian), NEUM
• Powhatan: OPOSSUM (op assom, "white dog")
• Russian: PRAESIDIUM, PRESIDIUM.
• Tamil: POPADUM, POPPADUM
• Urdu: BEGUM
• Yiddish: SCHTUM, SHTUM
• Nonsense words: BOOJUM (from The Hunting of the Snark), PANJANDRUM (from Great Panjandrum)
• Original coinages: COCKALORUM (elaboration of cock), FREEMIUM (from free + premium), PABLUM and PABULUM (from Latin pascere, "to feed"), PETROLATUM (from petrol + -atum), POSSUM (short for opossum), RUMDUM (US, from rum + dumb), SCRUM (short for scrummage)
• Unknown origin: HOODLUM, LABARUM (unknown origin, probably Late Latin), RUM (in both senses), SLUM
• Other: BLUEGUM, BUBBLEGUM, CHECKSUM, DRUM (Middle Dutch, Low German or Scottish Gaelic and Irish in various senses), DUMDUM (name of a town in India), EARDRUM, HUMDRUM (probable reduplication of hum), KETTLEDRUM, KHOUM (monetary unit of Mauritania), NGULTRUM (monetary unit of Bhutan, plural same), PRACTICUM (Late Latin)

Miscellany:-

• COLISEUM and COLOSSEUM (alteration from Medieval Latin, plural is COLISEUMS/COLOSSEUMS)
• CONUNDRUM (unknown origin)
• CUM (preposition)
• FACTOTUM (from Latin fac totum, "do the whole thing!". Plural is FACTOTUMS)
• FRAENULUM, FRAENUM, FRENULUM, FRENUM (Modern Latin, "little bridge". Plurals are FRAENULUMS etc.)
• JORUM (maybe named after Biblical character Joram)
• LOCUM (short for locum tenens)
• PRECORDIUM (inferred from Latin praecordia, "entrails")
• PREMIUM (from Latin praemium, "that which is taken before")
• QUORUM (from Latin quorum vos unum esse volumus, "of whom we wish that you be one")

To do

CEMENTUM

SOLANUM SORGHUM SPHAGNUM SPLENIUM SPUTUM STELLIUM STEPMUM STEWBUM STICKUM STRAMONIUM STRUM STUM STUMBLEBUM SUGARPLUM SUM SUPREMUM SWUM TALCUM TAPETUM TECTUM TEDIUM TEETOTUM THRUM TRIDUUM TRILLIUM TRITIUM TROPAEOLUM UROPYGIUM VALLUM VARIORUM VELLUM VERATRUM VERBASCUM VIBURNUM WAMPUM YESSUM.