The word trivial comes from Latin, and is originally a word for the kind of things discussed in a trivium, a crossroads where three roads meet.
In the Roman empire a trivium would often have a tavern (Latin: taverna) or similar, where trivial things could be discussed, as opposed to the things discussed in other locations.
In mathematics the term is frequently used for special cases that have a very simple structure, but for completeness sake can not be ignored. For non-mathematicians, they are often more difficult to understand than other simple cases. Examples include:
Mathematical use of the word trivial