English for Today by Ganiu Abisoye Bamgbose (Dr GAB)
IN SUIT or ON SUIT?
I saw you yesterday; you were ~on~ a blue suit (nonstandard).
I saw you yesterday; you were *in* a blue suit (standard).
Note that you are said to be in whatever you are wearing.
I saw him yesterday; he was putting ~in~ a blue suit (nonstandard).
I saw him yesterday; he was putting *on* a blue suit (standard).
While you are said to be in what you’re wearing, you are putting on same.
Note that as an idiom, “the men in suits” means the men who are in control of an organisation or company and who have a lot of power:
If you want a position in your organisation, do not offend the men in suits.
GAB
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