This is the 2189th most frequent German word.
bösen
"Bösen" is the plural form of "böse," which means "evil" or "wicked" in English.
The adjective 'bösen' is used to describe the negative or angry expression in 'Blick' (look).
Er hat einen bösen Blick geworfen.
He gave an angry look.
Here, 'bösen' describes the bad or wicked nature of the 'Drachen' (dragons) narrated.
Der Junge erzählt oft von bösen Drachen in seinen Geschichten.
The boy often talks about evil dragons in his stories.
In this case, 'bösen' qualifies 'Gedanken' (thoughts) as negative or undesired.