Then we associate the additional properties of the thing - which is bo, but also "rolls, transports and roars" - with the sound (and in writing: letter) r. And we call the whole thing a bor bo+r.
And now from the root one branch grew.
Other things are characterized by those properties that are true for boxes and cars, they all bo and r but also other characteristics are true to them. One type of them is big and can hold/transport many people. In English that thing is called "bus". But instead calling it a "boxcarbus" we - again - condense these additional properties into one letter or syllable: s or us. Then we unite this with the previously mentioned properties, by gluing it to their end: bors or borus, whatever you like, let's go with the latter.