Q: When “the scales fall from one’s eyes” to suddenly reveal the truth, are they the scales of justice?
A: No, the “scales” here are etymologically related to the ones on fish, reptiles, and insects.
The Oxford English Dictionary has three major meanings for the noun “scale,” with many related senses: (1) a device for weighing things; (2) one of the thin, overlapping plates protecting the skin of animals; (3) a graduated system of measurement.
When English borrowed “scale” from Old Norse in the 1100s, it meant a bowl or cup to drink from. The Old Norse skál is also the ultimate source of the drinking toast “skoal,” according to John Ayto’s Dictionary of Word Origins.
The OED’s earliest example for “scale” is from Layamon’s Brut, written sometime before 1200: “Heo fulde hir scale of wine” (“He filled her cup with wine”).
In the early 1400s, this cup-like sense of “scale” evolved to mean a device for weighing—that is, “the pan, or each of the pans, of a balance,” according to Oxford.
The earliest example in the OED for “scale” used in the weighing sense is from An Alphabet of Tales (circa 1440), a collection of moral stories:
“And when it was put In þe to skale it weyed more þan all þat evur þai cuthe put in þe toder skale” (“And when it was put into the scale, it weighed more than all that ever they could put in the other scale”). We’ve expanded the OED citation for clarity.
The dictionary’s first citation for “scales” used in the justice sense is from Christ’s Victory and Triumph (1610), an allegorical poem by Giles Fletcher: “In one hand a paire of euen scoals [even scales] she weares.”
The amphibian sense of “scale” is ultimately derived from skaljō, the ancient German source for the word “shell.” The earliest OED citation is a reference to the scales on a dragon, from Guy of Warwick, a Middle English romance dating from around 1330:
“Þe smallest scale þat on him is No wepen no may atame” (“No weapon may cut into the smallest scale that’s on him”).
And here’s a piscine example from the Chaucer poem Parlement of Foules (circa 1381): “Smale fischis lite / With fynnys rede & skalis syluyr bryȝt” (“Skinny little fishes / With red fins and bright scales”).
The OED’s earliest example for the expression you’ve asked about is from the Wycliffe Bible of 1382: “And anon ther felden from his yȝen as scalis, and he receyuede siȝt” (“And anon there fell scales from his eyes, and he received sight”).
Think of the expression as a metaphorical way of describing something akin to the sudden clearing of the cloudy layer on the lens of someone with cataracts.
The measurement sense of “scale” is derived from the classical Latin scāla (ladder) and scālae (flight of stairs). In the late 1300s and early 1400s, “scale” was used literally for a ladder and figuratively for a stair-like series of gradations for measuring things, according to OED citations.
It’s used literally in John Lydgate’s poem Troy Book (1412-20), where Diomedes says the Trojans fortified their walls while the Greeks delayed attacking “and ageyn oure skalis … made gret ordinaunce” (“and against our ladders … assembled a great defense”).
in A Treatise on the Astrolabe, a 1391 how-to manual about the astrological instrument, Chaucer uses the word figuratively: “Next the forseide cercle of the A. b. c., vnder the cros-lyne, is Marked the skale, in Maner of 2 Squyres or elles in Manere of laddres.”
Over the years, the noun “scale” has taken on many other meanings derived from the “ladder” sense of the word, including a “scale” in music (1597), doing something on a large or small “scale” (1785), a wage “scale” (1921), and economies of “scale” (1944).
from Grammarphobia http://www.grammarphobia.com/blog/2016/05/scales.html