The story
From -adic (taken from monadic/dyadic/triadic [function/operator]) + -ity, or alternatively from -ad (taken from monad/duad/triad) + -icity. Compare Latinate equivalent arity, based on -ary.
Etymology from Wiktionary, CC BY-SA 4.0.
adicity
Today it means: The number of arguments or operands a function or operation takes. For a relation, the number of domains in the corresponding Cartesian product. Full definition →