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 they do not.

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) 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, LAWRENCIUM, LUTETIUM, MEITNERIUM, MOLYBDENUM, NEODYMIUM, NEPTUNIUM, NIOBIUM, NOBELIUM, OSMIUM, PLATINUM, PLUTONIUM, POLONIUM, 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).

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, GERANIUM (secondary sense is a mass noun), GEUM, GUAIACUM, HELENIUM, HYPERICUM.

Not Latin: BEGUM (from Urdu), BLUEGUM (compound with GUM), BOOJUM (nonsense word from The Hunting of the Snark), BUBBLEGUM, CARDAMUM (from Old French cardamome), CHECKSUM, CHETRUM (from Dzongkha), CHILLUM (from Hindi), COCKALORUM (original coinage from cock), DIAZONIUM (from German), DRUM (from Middle Dutch, Low German or Scottish Gaelic and Irish), DUMDUM (name of a town in India), EARDRUM, EUPHONIUM (from Greek, given Latinate ending), FREEMIUM (original coinage from free + premium), GUM (from Old French gomme, ultimately from Egyptian), HARMONIUM (from French, Latin is harmonia), HOODLUM (unknown origin), HUMDRUM (probable reduplication of hum), KETTLEDRUM, KHOUM (monetary unit of Mauritania).

Noun in English but adjective in Latin: DECORUM, INFIMUM (lowest).

Noun in English but gerundive in Latin: DEDENDUM, HABENDUM, JEJUNUM (fasting).

Other:-

• ASYLUM (of Latin origin but for some reason plural is still ASYLUMS)
• 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)

To do

ALARUM (mistake?), ALUM (mistake?), CEMENTUM

LABARUM LABDANUM LABURNUM LADANUM LANTHANUM LAUDANUM LIGUSTRUM LINOLEUM LOCUM LOMENTUM LUTECIUM LYCOPODIUM MAGNUM MECONIUM MEERSCHAUM MENTUM MODICUM MUSEUM MYOCARDIUM MYOMETRIUM NASTURTIUM NATATORIUM NEUM NGULTRUM NOSTRUM OAKUM ODIUM OLEUM OLIBANUM OPIUM OPOSSUM OPPROBRIUM ORICHALCUM ORIGANUM PABLUM PABULUM PANJANDRUM PANTOUM PASPALUM PENDULUM PEPLUM PERIMYSIUM PETROLATUM PETROLEUM PHORMIUM PLENUM PLUM POLYGONUM POMATUM POPADUM POPPADUM POSSUM POTASSIUM PRACTICUM PRAESIDIUM PRECORDIUM PREMIUM PRESIDIUM PROTIUM PSALTERIUM PSYLLIUM PUERPERIUM PYRETHRUM QUADRIVIUM QUORUM RHIZOBIUM RUM RUMDUM SANCTUM SARGASSUM SCHTUM SCRUM SCUM SEBUM SEDUM SESAMUM SHTUM SKOOKUM SLUM 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.