
plump
crude
Tactless, coarse, and without sensitivity; in a very direct and awkward way.
Sein plumper Anmachspruch kam bei ihr gar nicht gut an.
His crude pickup line did not go over well with her at all.
crude / clumsy / bulky
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crude
Tactless, coarse, and without sensitivity; in a very direct and awkward way.
Sein plumper Anmachspruch kam bei ihr gar nicht gut an.
His crude pickup line did not go over well with her at all.

clumsy
Awkward, heavy, and lacking elegance in movement.
Der Bär bewegte sich plump durch das Unterholz.
The bear moved clumsily through the undergrowth.

bulky
Thick, shapeless, and chunky in appearance.
Für die filigrane Dekoration war der Stift viel zu plump.
The pen was far too bulky for the delicate decoration.
The German word 'plump' is negative and means awkward, tactless, or bulky. It should not be confused with the English 'plump', which means 'mollig' or 'rundlich' (chubby or rounded) and is often meant neutrally or even positively.
Seine plumpe Entschuldigung hat alles nur noch schlimmer gemacht.
His crude apology only made everything worse.
In the meaning 'taktlos' (tactless), 'plump' often describes communication or social behavior. Typical combinations are: a 'plumpe Lüge' (crude lie), a 'plumper Versuch' (crude attempt), a 'plumpe Ausrede' (crude excuse), or a 'plumpe Frage' (tactless question).
Das war ein ziemlich plumper Versuch, das Thema zu wechseln.
That was a pretty crude attempt to change the subject.
When referring to movements, 'plump' describes a heavy, inelegant manner. People often speak of 'plumpen Bewegungen' (clumsy movements) or say that someone or an animal 'sich plump bewegt' (moves clumsily).
Der Pinguin wirkt an Land plump, ist im Wasser aber sehr elegant.
The penguin looks clumsy on land, but is very elegant in the water.
When referring to objects, 'plump' means that something is bulky, thick, or shapeless. Examples are 'plumpe Schuhe' (bulky shoes), a 'plumpes Möbelstück' (bulky piece of furniture), or 'plumpe Schrift' (clumsy-looking handwriting/typeface).
Für die feine Gravur ist dieses Werkzeug viel zu plump.
This tool is far too bulky for the fine engraving.
impolite
Both words can describe a hurtful or bad way of dealing with other people. Unhöflich generally refers to a lack of good manners, while plump additionally emphasizes a crude, very direct, and awkward manner in a delicate situation. Use plump when the main issue is the lack of tact.

Seine plumpen Fragen nach ihrem Gehalt machten die Kollegin verlegen.
His crude questions about her salary made the colleague feel embarrassed.
Es war unhöflich, die Kollegin während ihres Gesprächs einfach zu unterbrechen.
It was impolite to simply interrupt the colleague while she was speaking.