Plural form of 'abacus'; multiple ancient counting devices made of beads or counters strung on rods, used for arithmetic.
From 'abacus' plus the English plural suffix '-es.' 'Abacus' itself comes from Greek 'abax' (a board or table), which may derive from Semitic languages. This is the standard English plural, while 'abaci' is the Latin plural.
The fact that we can call them 'abacuses' (English plural) or 'abaci' (Latin plural) shows how languages borrow words—when English borrowed 'abacus' from Latin, we kept fighting over whether to use the Latin or English plural, and both eventually became accepted.
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