If something is tenuous, then it is thin or tender, either literally or figuratively. It stems from the Latin "tenuis," meaning thin, drawn out, meagre, not having much substance.

In the literal sense for example, scientists say that the atmosphere around Pluto is tenuous. In other words it's thin, making the planet uninhabitable for humans. Ice on a body of water like a pond or a lake is tenuous if it's thin of course.

Speaking of thin ice, for those familiar with the phrase "on thin ice," meaning risky or precarious, don't try to impress your friends by changing the expression to "on tenuous ice" it would sound wierd. The general rule is, don't change familiar phrases.

The figurative sense of tenuous, which is the more common usage, is typically used to describe an opinion, position or argument as indefensible, weak or unsound:

"Your position on this matter is tenuous at best."

It's also used as a synonym for the figurative sense of shaky, meaning uncertain or questionable

"His immigration status is tenuous".

---
SYNONYMS

delicate, doubtful, insignificant, flimsy, lightweight, meagre, questionable, shaky, sketchy, uncertain, weak, thin.

---
SMUGGLE OWAD into today's conversation

"Our relationship was tenuous at first, but later we learned to work well together."