die Badehose
swimming trunks
A pair of shorts for men and boys worn for swimming and bathing.
Vergiss nicht, deine Badehose einzupacken, wenn wir ins Schwimmbad gehen.
Don't forget to pack your swimming trunks when we go to the swimming pool.

swimming trunks
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swimming trunks
A pair of shorts for men and boys worn for swimming and bathing.
Vergiss nicht, deine Badehose einzupacken, wenn wir ins Schwimmbad gehen.
Don't forget to pack your swimming trunks when we go to the swimming pool.

The word is made up of 'Bade-' (from the verb 'baden' (to bathe/swim)) and 'Hose' (trousers/pants). So it is literally a pair of trousers for bathing or swimming.
Für den Strandurlaub brauche ich noch eine neue Badehose.
I still need a new pair of swimming trunks for the beach vacation.
'Badehose' (swimming trunks) is often used with verbs like 'anziehen' (to put on), 'tragen' (to wear), 'einpacken' (to pack), or 'vergessen' (to forget).
Zieh dir schnell die Badehose an, wir wollen los!
Quickly put on your swimming trunks; we want to leave!
The word is used almost exclusively in the context of water and swimming, for example at the swimming pool, at a lake, or by the sea.
Im Hotelpool ist das Tragen einer Badehose Pflicht.
Wearing swimming trunks is required in the hotel pool.
bikini
'Bikini' and 'Badehose' both belong to clothing for swimming. The difference is simple: a 'Bikini' is a two-piece swimsuit for women. A 'Badehose' is a pair of shorts for swimming, usually for men and boys. Learners may confuse the words because both are worn at the beach or in the swimming pool. If you mean a top and a bottom as a set, 'Bikini' fits. If you mean only the shorts for swimming, 'Badehose' fits.

Er packt seine Badehose ein, weil die Klasse heute schwimmen geht.
He packs his swimming trunks because the class is going swimming today.
Für den Urlaub am Meer hat sie einen neuen Bikini gekauft.
She bought a new bikini for her vacation by the sea.