Recall that ‘wanting in’ is AGspeak for ‘lacking in common use.’
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Place Names
Athēnae (Athens), Thūriī (id.), Philippī (id.), Velī (id.)
Festivals
Olympia (n. pl. the Olympic Games), Bacchānālia (feasts of Bacchus), Quīnguātrūs (festival of Minerva), lūdī Rōmānī (the Roman Games)
Groups and Classes
optimātēs (the upper classes), maiōrēs (ancestors), līberī (children), Diī penātēs (household gods), Quiritēs (citizens), patrēs conscriptī (fathers conscript)
Other Words
arma (arms), artūs (limbs), dīvitiae (riches), scālae (stairs), forēs (double-doors), angustiae (narrow pass), moenia (city walls)
A few of these words are made singular in English…
dēliciae (darling), faucēs (throat), īnsidiae (ambush), cervīcēs (neck), viscera (flesh)
Exceptions
After this list, AG has a note more or less dismissing their classification, and admitting it’s more of a tendency. Indeed, optimās may be far more rare than optimātēs, but that doesn’t make the word ‘wanting in’. Likewise with artūs; there’s nothing odd about artus. Finally, they mention that scāla is a rare word for ladder—in case you’re curious.
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The Essential AG: 101