Expressing weakness of alleged reasons for something

Question

Let's take the sentence

Sentence. Jim was detained on a slim ground that he was seen with Bob two days ago.

The idea is to say that the justification for Jim's detention is weak, and I wanted to express it using the phrase 'on a slim ground'. However, having checked both google ngrams and google search, I found that the phrase 'on a slim ground' and its alternative 'on slim grounds' are not popular at all, so it is probably incorrect to use them.

Question. Is it true that the sentence at hand is not correct, and if so, how to replace 'on a slim ground' to make it correct?

Answer 1

The expression using the plural grounds is approximately twice as popular today as the singular ground.

However, Google Books Ngram Viewer indicates that detained on the ground is much older than the plural version and until the turn of the millennium, was also much more popular.

However, it's always detained on THE grounds and not on a ground.

So if you change the indefinite to the definite article, yes, your sentence is grammatical. As to the use of slim, it's one of a number of adjectives that could be used to indicate that there was little evidence to justify his detention.

https://books.google.com/ngrams/graph?content=detained+on+the+grounds+that%2Cdetained+on+the+ground+that&year