Why do people say the word "suspect" instead of "suspicious"?

Question

like "i'm suspect of him" instead of "i'm suspicious of him".

Answer 1

More  or less the same but only in usage i.e. the application to a correct text.

Answer 2

People seem to modify words...reducing the number of syllables. The fewer syllables, the better.  

Answer 3

It will be grammatically correct to say - I'm suspicious of him.

If you want to use 'suspect', the sentence has to be constructed this way - I suspect him.

One reason why people say 'suspect' could be because they do not know the correct form to use but would like to bring the message across; especially if English is not their first language.

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Answer 4

It's a form of language change called back-formation.  Strictly speaking, it's incorrect, but in casual speech it can be used.