Modifiers matching duck:

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

Simple categories matching aggressive:

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