Why does the phrase "as well as" mean "in addition to"?

Solution 1:

Well, not an easy one to find I must admit. But at least we can clarify that well is used in this phrase as an adverb:

We use as + adjective/adverb + as to make comparisons when the things we are comparing are equal in some way:

• The weather this summer is as bad as last year.
• You have to unwrap it as carefully as you can.

So we know that as well as has nothing to do with well as a noun ("a deep hole in the ground from which you can get water, oil, or gas" - Cambridge).

In most dictionaries, the origin of the phrase is either described as not accurate, or broken down to the etymology of each word, which actually does not help.

However, looking at the etymology of well, you see that it means:

well (adv.) - "in a satisfactory manner" (etymonline)

So it kind of makes sense that if something is as satisfactory or acceptable as something else, then it is added to the same class or category in the mind of the speaker. Whence, in addition (to).

Solution 2:

What is the etymology? It seems likely that it comes from the separate words as and well.

It does seem that when it meant in addition to, some people spelled as well with one word: aswell or aswel, between 1550 and 1650. See Google Ngrams. For example, see this sermon from 1612:

The vices aswell as vertues, the slippes and falles, aswell as the steadfast standinges, and upright walkinges; the criminations aswell as commendations, of Gods best servants ...

However, the OED has a citation for as well meaning in addition to in 1384, well before it started being compressed into one word. So the expression seems to have originally been two words: as well. For around a hundred years starting around 1550, some people spelled it with one word when it meant in addition to, but then the one-word spelling lost currency to the two-word spelling.

How did it take on this meaning? This is pure speculation, but looking at the early Middle English meanings of well, it's possible that as well as originally meant to the same extent as, which could have changed to in addition to. Here's a citation from the OED (much too late to demonstrate that this was the origin of the phrase, but it does show how this change in meaning could have taken place):

c1475: It behoueth to him..not only to haue his hondis and his tonge cloos but as well his yen.

If you take the meaning of "as well" to be "to the same extent as", this sentence becomes:

It behooves him not only to have his hands and his tongue closed, but just as much his eyes.

Solution 3:

I highly disagree that "aswell" was the archaic form of "as well". I believe it was just a misspelling of "as well". The phrase "as well" (with a space) has been around since the late 15th century.

as (adv., conj., pron.)

c. 1200, worn-down form of Old English alswa "quite so, wholly so," literally "all so" (see also), fully established by c. 1400. Equivalent to so; any distinction in use is purely idiomatic. Related to German als "as, than," from Middle High German also.

Phrase as well "just as much" is recorded from late 15c.; the phrase also can imply "as well as not," "as well as anything else." Phrase as if, in Kantian metaphysics (translating German als ob), introducing a supposition not to be taken literally, is from 1892; as an interjection of incredulity (as if!; i.e. "as if that really could happen") is attested from 1995. It duplicates Latin quasi. Phrase as it were "as if it were so" is attested from late 14c.

(Etymonline)

From Merriam Webster:

First Known Use of as well as

Conjunction

15th century, in the meaning defined above

Preposition

1589, in the meaning defined above