Q:

On the origin of "casa"

I usually find that the etymologies of the simplest words are often the most interesting ones. In Spanish the word casa 'house' derives from Latin căsa, ae which had the following meaning:

any simple or poorly-built house, a cottage, hut, cabin, shed, etc.

Nonetheless, the word casa is translated into Latin as domus, as seen in the Nebrija dictionary from 1495:

Caſa por el edificio. domus.us. ȩdes.ium

So the standard word for "house" was domus in Latin but the Spanish language adopted casa, which was just a "poor house", as the standard word for that concept and used since the 10th century until today. The Latin word domus remains in Spanish in derived words such as domicilio.
I just wonder, what were the historical reasons that made the Spanish language adopt such a word as the standard word for "house" instead of an evolved form of domus? Note that Spanish casa also conveys the meaning of "home" and not only the one related to the building itself, maybe this had something to do with the process?

A:

This section from Harrington's Medieval Latin details this phenomenon quite well. It also contains an extensive list of similar displacements at the end of the chapter.

2. VOCABULARY CHANGES
Further difficulties of ML are that familiar words have changed their meaning, and that a perhaps less familiar word has replaced a better-known one. Löfstedt 1, 340, writes:

“It is a characteristic feature of Latin in its later stages that a good many words well-known or seemingly well-known from earlier periods occur with new and surprising meanings. . . . The change is, in some cases, a natural and organic one due to special circumstances involving an inner change of meaning; in other cases it is more correct to consider it a sort of etymologizing new formation either learned or popular in origin or resting on ignorance (it is not always easy to decide which).”

Changes often involve the status of a word, as a term of humble origin replaces a more elevated one (i.e., CASA, “hut,” replaces DOMUS, “house”). A phenomenon common to the development of all languages is the restriction of a word of general meaning to a specific use or the reverse (e.g., TESTIMONIUM, “evidence” becomes TESTIS, “witness” [Löfstedt 2, 15 1-5 2]). Another example of the specific replacing the general: MACHINA, “invention,” “artifice” > “machine.” Furthermore, seemingly irregular formations (e.g., verbs With reduplicated perfects) tend to be replaced by more regular (and first conjugation) forms (CANO, CANERE, CECINI is replaced by CANTO, CANTARE, CANTAVI).
By constant use, moreover, the meanings of words seem to erode. Simple verbs, e.g., DO, DARE, DEDI, with its monosyllabic forms and reduplicated perfect, were replaced by the frequentative, DONARE (cf. Fr. donner); frequentatives, moreover, had the additional advantage of seeming stronger than the simple forms (Löfstedt 2, 28). To give another example, AUSARE replaced AUDERE (It. osare, Fr. 0567', Sp. osar).
Monosyllabic verb forms tended to be replaced — i.e., is, it from ire were replaced by compounds (inire, exire), or by other verbs such as ambulare and vadere; fles, flet (from flere) were replaced by forms of ploro or plango; flas, flat ( from flare) were replaced by the compound sufflare (> Fr. souffler); es, est (from edo) were replaced by forms of comedo (or manducare). This process of compounding, moreover, was not limited to monosyllabic forms: cf. expandere replacing pandere, adimplere replacing implere.
For nouns, the fourth and fifth declensions disappeared from the spoken language, fourth declension nouns being assimilated to the second, and fifth to the third (sec §§ 3.1 and 3.2). In addition, many diminutives, which belong to the first or second declension, replaced the customary CL form (e.g., SOLICULUM may replace SOL, sous (cf. Fr. soleil, but It. sole < SOL).
Sound was yet another factor which influenced the vocabulary of Medieval Latin. It has been noted several times that words with similar sound clusters and adjacent senses influenced one another (Löfstedt and Norberg, cited by Westerbergh 289—90). For example, in the Chronicon Salemitanum, expectare (to expect) aliquid ab aliquo came to mean expetere (to demand) aliquid ab aliquo.
Finally, many Greek loanwords displaced the CL forms, as COLAPHUS replaced ICTUS (4th) (cf. It. colpo, Fr. coup, Sp. golpe); THIUS in Italy and Spain appears to have supplanted AVUNCULUS (It. zio, Sp. tío), while in France AVUNCULUS remained in use (oncle). C(H)ORDA replaced the deceptive FUNIS (3d).

