This is how I use these highly technical terms when describing the complexity of a bit of design or development work.

Trivial:

Easy (though it will still take half a day even though it should take half and hour). I try not to ever quote less than half a day for a piece of work.

Non-trivial:

The exact opposite of trivial, i.e. "It's going to be hard, but I'll do it if you make me, just so long as you know there are better things I could be doing with my life"

Slightly non-trivial:

Depending the tone of my voice when I say it, either: "Just a bit easier than non-trivial" OR "damn near impossible".

English is an odd language.