
die Schlappe
defeat / setback
A clear defeat, failure, or setback, often in a competition or conflict.
Die Partei musste bei der Wahl eine herbe Schlappe einstecken.
The party had to suffer a severe defeat in the election.
defeat / slipper
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defeat / setback
A clear defeat, failure, or setback, often in a competition or conflict.
Die Partei musste bei der Wahl eine herbe Schlappe einstecken.
The party had to suffer a severe defeat in the election.

slipper
A simple, open house slipper or bathing sandal that you can easily slip into. Often used in the plural ('Schlappen').
Ich finde meine linke Schlappe nicht.
I can't find my left slipper.
'Die Schlappe' is related to the adjective 'schlapp' (weak, tired). A defeat ('Schlappe') makes you 'schlapp', and a house slipper ('Schlappe') is a 'schlapper' shoe, meaning a soft and shapeless shoe.
Nach dieser herben Schlappe fühlte sich die ganze Mannschaft schlapp.
After this severe defeat, the whole team felt weak.
You can easily recognize the meaning from the context. If it is about competition, sports, or politics, a defeat is meant. If it is about clothing, feet, or the home, the shoe is meant, which is often used in the plural ('Schlappen').
Die Regierung erlitt eine schwere Schlappe bei der Abstimmung.
The government suffered a serious defeat in the vote.
For the meaning 'Niederlage' (defeat), there are fixed verb combinations such as 'eine Schlappe einstecken' (to suffer a defeat), 'eine Schlappe erleiden' (to suffer a defeat), or 'eine Schlappe kassieren' (to take a defeat). It is often accompanied by adjectives like 'herb' (severe), 'bitter' (bitter), or 'deutlich' (clear).
Der Favorit musste im Finale eine unerwartete Schlappe einstecken.
The favorite had to suffer an unexpected defeat in the final.
defeat
Niederlage neutrally describes the fact that a game or duel was lost. Schlappe sounds more colloquial and usually presents the result as clear or embarrassing. Niederlage fits a factual report; Schlappe fits an emphatic assessment of a clear defeat.

Die Fans nannten das peinliche 0:5 eine schwere Schlappe.
The fans called the embarrassing 5–0 loss a heavy defeat.
Der Trainer sprach nach dem knappen 1:2 sachlich von einer Niederlage.
After the narrow 2–1 loss, the coach neutrally referred to it as a defeat.