Ramban on Gen 1:1, s.v. בראשית (the tohu–bohu paragraph)
This substance, which the Greeks called hyly, is called in the sacred language tohu, the word being derived from the expression of the Sages: “betohei (when the wicked bethinks himself) of his doings in the past.” If a person wants to decide a name for it [this primordial matter], he may bethink himself, change his mind and call it by another name since it has taken on no form to which the name should be attached. The form which this substance finally takes on is called in the sacred language bohu, which is a composite word made up of the two words bo hu (in it there is [substance]). This may be compared to the verse, Thou art not able ‘asohu’ (to perform it), in which case the word asohu is missing a vav and an aleph [and is a composite of the two words] aso hu. It is this which Scripture says, And he shall stretch over it the line of ‘tohu’ (confusion) and the stones of ‘bohu.’ [The tohu in Hebrew or hyly in Greek] is the line by which the craftsman delineates the plan of his structure and that which he hopes to make. This is derived from the expression, Kavei (Hope) unto G-d. The stones are forms in the building. Similarly it is written, They are accounted by Him as nought and ‘tohu,’ as tohu comes after nothingness and there is nothing yet in it. So the Rabbis have also said in Sefer Yetzirah: “He created substance from tohu, and made that which was nothing something.” They have furthermore said in the Midrash of Rabbi Nechunya ben Hakanah: “Rabbi Berachyah said: ‘What is the meaning of the verse, And the earth was ‘tohu’ (without form) ‘vavohu’ (and void)? What is the meaning of the word “was?” It had already been tohu. And what is tohu? It is a thing which astonishes people. It was then turned into bohu. And what is bohu? It is a thing which has substance, as it is written, [bohu is a composite of the two words] ‘bo hu’ (in it there is substance)
Charles B. Chavel, 1971–76 · chavel-ramban
Ramban on Gen 1:1, s.v. בראשית (the tohu–bohu paragraph) · sefaria-vocalized
The one witness on either bench that makes the pair a matched technical vocabulary. The primary matter, which the Greeks call hyle, is called in the holy tongue tohu — the word derived from the rabbinic betohe, of a man who bethinks himself of what he has done, because if a man came to fix a name to this matter he would think again and call it something else, having no form to fasten a name on. The form that the matter puts on is called bohu, a compound word, bo hu, 'in it there is'. Then Isa 34:11 read as a workshop: the line of tohu is the cord by which the craftsman marks out the plan of his building, the stones of bohu are the forms in it — the same verse the Chagigah baraita read as a green line round the world and stones in the deep. He closes with Sefer Yetzirah and with the Bahir's R. Berekhya, who defines tohu as 'a thing which astonishes people' — Rashi's etymology, now describing a substance rather than a viewer. Chavel's English shows on Sefaria as CC BY, which is surprising for a 1971 Shilo text: licence key chavel-ramban is marked for checking before publication. The paragraph that precedes this one — creation from absolute nothing, and the hyle as the only thing created — belongs to K4 and is not sliced here.