JD Ahmanson asked about the word scale because

• it can mean to climb up a cliff
• it can mean a device for showing how much something weighs, or
• it can refer to the outside of a fish.

The short answer is polysemy.

Erin McKean defined polysemy as “the greedy habit that some words have of taking more than one meaning for themselves.”

In the case of scale polysemy isn’t really the long answer because scale wasn’t a single word that adopted many meanings, it used to be several different words that came together in sound and spelling.  According to Merriam-Webster this is a homograph, a word with identical spelling and pronunciation that actually evolved from different sources and so holds more than one meaning.

So let’s explore the three meanings JD mentioned.

Scale “to climb” appeared in English in the 1400s from Italian (a refreshing change).

Since Italian used to be Latin we aren’t surprised that the ultimate etymology was scandere “to climb.”  When it first came into English it actually meant “a ladder” or “set of stairs.”

It’s easy to see how this word also got applied to various things that went up, like scales in music or the Richter scale that measures earthquakes.

It might make sense that if the Richter scale came from this ladder or climbing meaning, scales that measure weight could also have come from the same source.

But it seems that they don’t.

Weigh scales come instead from an Old Norse word skal that meant “a drinking bowl.”  This is also the source of our word skol that you might say when drinking a toast.

So weigh scales came about because they are made with two pans or bowls, one hanging on each side.

It seems to me though that the fact that the Richter scale measures, and weigh scales measure might have well influenced the evolution of these two different words into a homograph.

The “drinking bowl” meaning shows up first in 1205 in English and by 1375 the “weigh scale” had arrived.

Finally we turn to fish scales.

Here I can actually fall back on that polysemy word because in some ways fish scales did diverge from one of the words I’ve already covered.  But that was a very long time ago.

The American Heritage Dictionary points back to an Indo-European root skel meaning “to cut.” This is the root of shell.

You can imagine ancient people using shells to cut things.

What is a fish scale but a small shell-like plate?  Actually what is a drinking bowl but a large deep shell?

The fish scale etymology got to English around 1300 through French, but the French picked it up from Germanic.

The drinking bowl etymology shows the word also coming from Germanic but from the north with Old Norse instead of the south with French.  So these two meanings—fish scale and bathroom scale—started in the same place, diverged, then came together in spelling and pronunciation, but not in meaning.

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