Technically they can. I could register whatever sequence of letters I want provided they can act as a unique identifier for me, and are not in themselves descriptive. If I register 'Bolt' for dog-walking services this is not descriptive and provided I pay my renewal fees I can protect the word forever, and take action against those whose use of it confuses users (for example 'Bolt' for dog food) or takes unfair advantage of the reputation in my mark.

The presence of a word in a dictionary/its length has no bearing on its registrability as a trade mark.