Learner's definition of SUSPICIOUS
[more suspicious; most suspicious]
1 : causing a feeling that something is wrong or that someone is behaving wrongly : causing suspicion
We were instructed to report any suspicious activity/behavior in the neighborhood.
The suspicious vehicle was reported to police.
He died under suspicious circumstances. [=under circumstances that suggest a crime may have been committed]
Suspicious characters were seen hanging around the bank.
He found a suspicious lump on his back and was afraid it might be cancer.
2 : having or showing a feeling that something is wrong or that someone is behaving wrongly : feeling or showing suspicion — often + of
She became suspicious of his behavior.
The dog is suspicious of strangers. [=the dog does not trust strangers]
We noticed the suspicious stare of the security guard.