¿Por qué es la palabra «mano» femenina?

Question

En español, tenemos una regla en la cual, generalmente, se puede tener fe: Si una palabra termina con -o, es masculina. Sin embargo, palabras que terminan en -e o -a también pueden ser palabras masculinas, pero una que termina con -o es masculina sin duda.

Existe la excepción en la palabra «mano». ¿Por qué es esto? ¿Cómo se originó esta excepción?

Answer 1

Gender

While the rule is in general that all -o words are masculine, there are exceptions. This one is one of these exceptions to that rule.

Origin

One theory (proposed here) is that this word derived from the Latin word manus which was a fourth-declension feminine noun. For comparison, first declension nouns were all feminine and second declension were all masculine or neuter. This particular word, though, was a fourth-declension noun.

Fourth declension nouns were a bit different since they are predominantly masculine with some feminine exceptions. "Manus" was one of those exceptions.

Summary

More than likely, the word la mano derived from the Latin word manus, which was feminine.

Answer 2

1. Why most nouns ending -o are masculine, and -a are feminine

Spanish is a Romance language, which means it is descended from Latin. In Latin, nouns fell into 5 groups, depending on how they were declined (i.e. how their suffixes changed depending on case).

The first declension nouns usually end in -a in the nominative singular and are mostly feminine, which is why most words ending -a are feminine in Spanish.

The second and fourth declension nouns usually end -us in the nominative singular and are mostly masculine. Latin words ending -us generally evolved into words ending -o in Spanish, which is why most words ending -o are masculine in Spanish.

2. Why mano bucks the trend

mano comes from Latin manus - one of the few feminine fourth declension nouns, which explains why it ends -o.

However, even within these feminine 4th declension nouns, manus is an exception in Spanish, being the only one to both retain its feminine gender and its (morphologically) expected suffix:

Changed gender

• echo > eco (m)
• porticus > pórtico (m)
• īdūs (pl) > idus (m, pl)
• quīnquātrūs (pl) > quinquatro (m)²
• lat domus > it duomo > fr dôme > es domo (m)

Changed suffix

• fagus > [materia] fāgea > faya > haya (f)
• acus > *acūcula > *acūcla > aguja (f)
• socrus > *socra¹ > suegra (f)
• nurus > *nora > nuera (f)
• tribus > tribu (f)

Both

• quercus > quernus > al quernus > al quernus + occus > alcornoque (m)

Neither

• manus > mano (f)

The reason mano maintained both of these aspects is probably due to a combination of factors:

• it didn't refer to a female person, and so had no semantic pressure to change (e.g. nuera, suegra)
• it was a very common word
• it didn't gain a diminutive suffix in its evolution
• it didn't refer to a tree³

---

Notes

^{1. "Father-in-law" was socer in Latin, which evolved expectedly from the singular accusitive socrum > *socro > suegro, which may have also influenced socrus becoming suegra.}

^{2. 1803 DLE.}

^{3. In addition to the examples presented, fourth declension nouns referring to trees were feminine in Latin as well. These almost exclusively became masculine in Spanish, with the exception of the above fagus > haya:
• pinus > pino (m)
• taxus > tejo (m)
• alnus > alno (m)
• prunus > pruno (m)
• populus > *plōppus > pobo, chopo (m) etc}