I am trying to figure out when to use thin and when to use tenuous. It seems to me that the usage of tenuous is more limited, for example I would say "thin atmosphere" or "tenuous atmosphere", but only "thin man". It also seems to me that tenuous is more used in the sense of "delicate, slight" or "insecure" as in "tenuous position". Is there something like a rule of thumb, or is this a matter of language use that you simply have to get used to and learn?
2See the article in Merriam-Webster's Dictionary of Synonyms (an excellent resource for this kind of question).– Ben KovitzMay 31, 2017 at 19:24
1If you can literally put your thumb on it, it is probably not tenuous.– TᴚoɯɐuoJun 1, 2017 at 12:05
@Ben Kovitz Thanks, especially in view of the fact that I don't have to rob a bank for that book for a change.– mondegreen dispenserJun 1, 2017 at 13:08
@Tᴚoɯɐuo Yes, after reading a perceived million example sentences on vocabulary.com today I think I got it.– mondegreen dispenserJun 1, 2017 at 13:11
1Most examples of tenuous fabric for the past 100 years have not been about literal cloth "fabric" but metaphorical, as in the "fabric of an argument", say, or the "fabric of reality" or the "fabric of international relations", though you will find some examples where the author is talking about a veil or negligee.– TᴚoɯɐuoJun 1, 2017 at 13:24
"Tenuous" means insubstantial or flimsy. You might say that the threads holding a piece of cloth together are tenuous, or that the air on Mars is tenuous, or that the form of a ghost is tenuous.
"Thin" refers to size. Someone who is 6 feet tall but weighs only 100 pounds is thin. A wooden beam that is 20 feet long but only 1 foot wide is thin. Etc.
"Thin" can be used to mean "tenuous". Like you can say, "The air on Mars is thin" and that would mean pretty much the same thing as "The air on Mars is tenuous." But "tenuous" cannot generally be used to mean "thin" in the sense of small dimensions. You can use it when the dimensions are so small that the item becomes fragile, like "the weight was suspended by one tenuous thread". But you can't say "Sally is a tenuous person" meaning that she is the opposite of fat.
Tenuous suggests something is uncertain and changeable, as well as being thin or rare - the tenuous atmosphere suggests an atmosphere that may not be there from moment to moment, or his tenuous position at the top suggests that he may lose that position with little warning. My tenuous grasp on reality is delicate and fragile, and the slightest stress could destroy it.
Thin does not have that same implication of changeability. A thin atmosphere is less dense, but still very definitely there, a thin margin for error is always the same.