Focus

Imagine you are walking down a cold winter road in central Gaul late one afternoon during the declining years of the Roman Empire. You’re on your way to visit a friend. Even with a woollen robe over your toga, you’re cold. Maybe you shouldn’t have worn the sandals? You arrive at your friend’s villa and bang on the gate. A moment later your friend opens the gate and says warmly, “Gratam mea! Quaso intrant et sedent focus!”
 
Which is to say, roughly, “Welcome to my home! Please enter. Come and sit by the fireplace!”
 
Which is to say that the word focus comes from the Latin word focus (fireplace, hearth). Beyond this, the origin of the word is unknown. The focus—the source of heat and light in a home—or a classroom for that matter. That’s all you need to know about the word.
 
There are other uses and meanings of the word focus which have come to English. In the 1640s, the word was used to mean ‘point of convergence’. At that time, focus also referred to the burning point of a lens. By 1796, focus was used to mean the centre of activity or energy.
 
Reference: Online Etymological Dictionary, https://www.etymonline.com/