I have been meaning to do this word forever. And now here it is.
Many words that have to do with balance and similarity start with equa-, so it shouldn’t surprise anyone that equal comes from the classical Latin aequalis, which means identical or uniform. There isn’t much information about further origins, just that it’s related to aequus, which means fair or even, as well as egalitarian by way of Old French.
Most of the other equa- words have similar origins. They were either taken directly from the classical Latin or from the Old French, which was in turn taken from classical Latin ; ). The first ones to show up in English were equity and iniquity. It’s worth noting that at first, iniquity just meant in- as in not and equity as in equal, so unequal. The evil meaning came later and since it’s not in the Latin version of the word, it was probably the Old French version that created the meaning. The whole change makes sense since there is no greater evil than injustice.
Also interesting is equinox and equator, both of which stem from Medieval Latin. The first comes from equinoxium, which means “equality of night and day” and is from aequus and nox (night). The second is from the phrase aequator diei et noctis, “equalizer of day and night”. I guess it was a little wordy because they shortened it when first referring to the diameter of Earth equidistant between the poles.
Of course, all the words have their stories. Equivocation, equivalent, even adequate, which originally meant “to make equal to” before it morphed into “equal to what is required.” Why did it change? Because we needed it to.