Tenuous
Etymology
From Latin “tenuis,” thin or slender, from the root “ten-,” to stretch (the same family as “tension” and “tendon”). A physically fine thread became a figure for anything stretched too thin to be reliable — a tenuous link, a tenuous truce.
adjective · /TEN-yoo-uhs/
So thin or weak as to be barely holding together.
What it names
Fragile support — a connection, claim, or grip that exists but could give way at the slightest strain.
Example
The theory rests on a tenuous connection between two events that may have nothing to do with each other.
Related idea
A tenuous claim is weak but may be true; a specious one looks strong but is false. Tenuous is honestly thin; specious is deceptively solid.