der Penner
bum
A derogatory and colloquial term for a homeless person who lives on the street.
Am Bahnhof saßen ein paar Penner und tranken Bier.
A few bums were sitting at the train station drinking beer.

bum / loser
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bum
A derogatory and colloquial term for a homeless person who lives on the street.
Am Bahnhof saßen ein paar Penner und tranken Bier.
A few bums were sitting at the train station drinking beer.

'Penner' is a very negative and insulting swear word. It is purely colloquial and should definitely be avoided by German learners so as not to hurt anyone.
Er wurde wütend und beschimpfte den Fahrer als „Penner“.
He got angry and insulted the driver by calling him a 'Penner' (bum/loser).
The word is derived from the colloquial verb 'pennen', which means 'to sleep'. So a 'Penner' is literally someone who sleeps somewhere, often outdoors.
Nach der langen Wanderung wollen wir heute Nacht nur noch pennen.
After the long hike, all we want to do tonight is sleep.
The word can refer directly to a homeless person or be used as a general insult for a person seen as lazy or unkempt, regardless of their housing situation.
„Steh auf, du fauler Penner!“, rief sein Mitbewohner.
'Get up, you lazy bum!' his roommate shouted.
loser
'der Loser' and 'der Penner' can both be used as harsh insults, but they have different focuses. 'Loser' means a person who is seen as unsuccessful or as a loser. 'Penner' is even cruder and can refer derogatorily to a homeless person, or more generally to someone considered lazy, unkempt, or incompetent. The words can be confused because both occur as insults in arguments. Still, you should be careful: 'Penner' is especially hurtful, particularly when it refers to homelessness.

Der Mann wurde wütend, weil jemand ihn auf der Straße einen Penner nannte.
The man got angry because someone on the street called him a bum.
Nach dem verlorenen Spiel fühlte sich Tom wie ein Loser.
After the lost game, Tom felt like a loser.