Pelo (Spanish for “hair”) is a surprisingly militaristic word. Pelo comes from the Latin for the same, pilus–a hairy word, indeed.
But pelo, in the ancient language become a common word to mean a tiny amount, like we might say a “spec” in modern English. Apparently, the Romans lost their hair early!
So, as a euphemism for “a tiny amount”, it became the standard word in Latin for… a small group of soldiers: a pilum.
Then, over the centuries, the word for a group of soldiers came to mean the word for… fighting. Surprise, surprise. Therefore, that’s why the Spanish for “to fight” is… pelear.
Thus pelo (“hair”) and pelear (“to fight”) are almost the same word, in Spanish! Who would’ve thunk!