This is the 12659th most frequent Polish word.
niechlubną
"Niechlubną" means "disgraceful" or "dishonorable."
In this sentence, 'niechlubną' is used as an adjective qualifying 'kartę' (chapter), describing it as something lacking in honor, hence shameful.
Historia tego regionu ma swoją niechlubną kartę.
The history of this region has its shameful chapter.
Here, 'niechlubną' describes 'opinię' (impression), signifying that the impression was disgraceful or marred by dishonor.
Jego zachowanie na spotkaniu pozostawiło niechlubną opinię wśród kolegów.
His behavior at the meeting left a shameful impression among colleagues.
In this example, 'niechlubną' modifies 'prawdą' (truth), indicating the truth is dishonorable or imbued with regret.