Modifiers matching duck:

• small,
• wild,
• tenacious,
• boastful,
• jealous,
• daffy,
• optimistic,
• lisping,
• brash,
• large,
• native,
• insecure,
• paranoid,
• breeding,
• determined,
• zany,
• aggressive,
• bullying,
• unusual,
• pushy,
• short-tempered,
• common,
• here,
• rare,
• smaller,
• domestic,
• indigenous,
• marine,
• slobbery,
• larger,

Simple categories matching aggressive:

• hacker,
• placental,
• person,
• behaviour,
• warrior,
• act,
• animal,
• crime,
• solvent,
• fish,
• thug,
• predator,
• hero,
• businessman,
• dictator,
• conflict,
• tyrant,
• demon,
• form,
• politician,
• shrub,
• liquid,
• change,
• monster,
• encounter,
• reagent,
• outdoorsman,
• pet,
• invertebrate,
• lady,
• option,
• bully,
• activist,
• campaigner,
• ceo,
• motive,
• treatment,
• condition,
• outlet,
• actor,
• tycoon,
• offense,
• president,
• disease,
• environment,
• creature,
• behavior,
• cargo,
• intervention,
• hunter,
• beast,
• king,
• illness,
• lesion,
• measure,
• grass,
• mayor,
• lymphoma,
• dog,
• athletics,
• villain,
• compound,
• duck,
• gas,
• explorer,
• activity,
• feeder,
• care,
• malignancy,
• agent,
• policeman,
• bass,
• womanizer,
• plant,
• humorist,
• cleaners,
• sportsman,
• superhero,
• pioneer,
• father,
• ant,
• dissident,
• native,
• cowboy,
• sport,
• tumor,
• interaction,
• acid,
• adventurer,
• emotion,
• fluid,
• caveman,
• vegetable,
• chemical,
• solution,
• emperor,
• tree,
• crooner,
• tetra,
• boxer,
• action,
• comedian,
• socialization,
• substance,
• spy,
• cop,
• scientist,
• cancer,
• terrorist,
• investor,
• nature,
• track,
• billionaire,
• soldier,
• material,
• detective,
• conqueror,
• ion,
• therapy,
• equine,
• insect,
• bird,
• game,
• medium,
• neoplasm,
• chef,
• offence,
• prey,
• child,
• interference,
• component,
• pest,
• cichlid,
• enzyme,
• athlete,
• event,
• singer,