dreckig
dirty
Covered with dirt, dust, or grime; not clean.
Nach dem Fußballspielen im Regen war seine Kleidung komplett dreckig.
After playing soccer in the rain, his clothes were completely dirty.

dirty
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dirty
Covered with dirt, dust, or grime; not clean.
Nach dem Fußballspielen im Regen war seine Kleidung komplett dreckig.
After playing soccer in the rain, his clothes were completely dirty.

The adjective 'dreckig' (dirty) is derived directly from the noun 'der Dreck' (dirt/grime), which means dirt or filth. It is the quality of being covered with dirt.
Zieh deine Schuhe aus, sie sind voller Dreck.
Take off your shoes; they are full of dirt.
In a figurative sense, 'dreckig' (dirty) describes not only physical dirt, but also morally bad or unfair behavior. Typical combinations are 'dreckige Witze' (dirty jokes), 'dreckige Tricks' (dirty tricks), or 'ein dreckiges Geschäft' (a dirty business/deal).
Er hat mit einem dreckigen Trick gewonnen.
He won with a dirty trick.
The expression 'sich dreckig fühlen' (to feel awful) or 'es geht mir dreckig' (I feel awful) is colloquial and means that someone feels very bad physically or emotionally, e.g., because of illness or sadness.
Nach der Trennung ging es ihm wochenlang richtig dreckig.
After the breakup, he felt really awful for weeks.
The fixed combination 'dreckig lachen' (to laugh dirtily) describes a laugh that does not sound cheerful, but rather maliciously pleased, mean, or suggestive.
Als er sah, dass ich hingefallen war, lachte er dreckig.
When he saw that I had fallen down, he laughed dirtily.
pure / in
'rein' and 'dreckig' are opposites when it comes to cleanliness. 'rein' means that something is clean and has no dirt. 'dreckig' means that something is covered with dirt or grime. 'dreckig' often sounds a bit stronger and more colloquial than 'schmutzig'. It is used especially when something is visibly or unpleasantly dirty. The contrast is simple: after washing, something can be 'rein'; after gardening, it is often 'dreckig'.

Wegen des Regens sind meine Schuhe nach dem Spaziergang dreckig.
Because of the rain, my shoes are dirty after the walk.
Für das Baby nimmt sie nur reine Handtücher aus dem Schrank.
For the baby, she only takes clean towels from the cupboard.