The Answer
The unit second is the “second diminished part” of an hour or circle (i.e. the second time it was divided by 60), which means that the numerical adjective came first. In fact, it was used nearly two centuries earlier.
Second is a very versatile word. It can be an adjective (second youngest child), ordinal number (2nd), a noun (1/60th of a minute or degree), a verb (second that emotion), another verb (second the officer to our unit) among others.
Let’s look at the origins of two of these words in particular to find out which came first. A second of time doesn’t appear to be related to the second that is number 2 in a series, but things are not always as they seem.
The Online Etymology Dictionary tells us that the original first use of the English word second is from around 1300, from Old French second, secont, and directly from Latin secundus “following, next in time or order”. This word derives from the Proto-Indo-European *sekw-ondo-, pariticipal form of root *sekw- “to follow.”
The origin of the noun second that describes 1/60th of a minute is explained in the following way,
“one-sixtieth of a minute of degree,” also “sixtieth part of a minute of time,” late 14c. in geometry, from Old French seconde, from Medieval Latin secunda, short for secunda pars minuta “second diminished part,” the result of the second division of the hour by sixty (the first being the “prime minute,” now called the minute), from Latin secunda, fem. of secundus “following, next in time or order”
So, the numerical meaning of the word second precedes the measurement by nearly two centuries.