Tangential
Etymology
From Latin "tangēns," the present participle of "tangere" ("to touch"). In geometry a tangent is a line that touches a curve at a single point and then departs. English borrowed that image for anything that makes only glancing contact with the topic before heading elsewhere.
adjective · /tan-JEN-shul/
Only slightly connected to the matter at hand; diverging from the main point.
What it names
A remark or detail that touches the subject and then veers off — the tangential aside is related enough to seem relevant but leads away from the core.
Example
His answer started on topic but wandered into a tangential story about his old car.
Related idea
The opposite of germane: a germane point bears squarely on the matter, while a tangential one merely brushes it before spinning away.