
das Mäuschen
little mouse
The diminutive form of Maus; a small mouse.
Ein kleines Mäuschen huschte schnell über den Küchenboden.
A little mouse quickly scurried across the kitchen floor.
little mouse / sweetheart
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little mouse
The diminutive form of Maus; a small mouse.
Ein kleines Mäuschen huschte schnell über den Küchenboden.
A little mouse quickly scurried across the kitchen floor.

sweetheart
A term of endearment for a person, especially for a woman or a child, expressing affection.
Komm her, mein Mäuschen, und gib mir einen Kuss.
Come here, my sweetheart, and give me a kiss.

to be a fly on the wall
An idiom expressing the wish to be able to listen in on or watch a conversation or event unnoticed.
Bei dem Treffen der Präsidenten hätte ich gerne Mäuschen gespielt.
I would have loved to be a fly on the wall at the meeting of the presidents.
'Mäuschen' (little mouse) is the diminutive form of 'die Maus' (the mouse). The ending '-chen' not only makes the noun small and cute, but also always changes the article to 'das'.
Hinter dem Schrank hat sich ein kleines Mäuschen versteckt.
A little mouse has hidden behind the cabinet.
You can recognize the meaning from the context. If people are talking about a small animal, the literal meaning is meant. If someone is addressing a person affectionately, it is a term of endearment. The fixed expression 'Mäuschen spielen' means always wanting to watch secretly.
Sei still, ich glaube, da ist ein Mäuschen in der Wand.
Be quiet, I think there is a little mouse in the wall.
As a term of endearment, 'Mäuschen' (sweetheart/little mouse) is used for loved ones, often for partners or children. It frequently appears together with a possessive pronoun such as 'mein' or 'meine'.
Keine Sorge, mein Mäuschen, wir schaffen das zusammen.
Don't worry, my sweetheart, we'll get through this together.
The idiom 'Mäuschen spielen' is a fixed unit. It expresses the wish to be present unnoticed in a private or secret situation. It is often used with 'möchte' (would like) or 'würde gerne' (would like to).
Wenn die Chefin mit dem Personalrat verhandelt, würde ich gerne Mäuschen spielen.
When the boss negotiates with the staff council, I would like to be a fly on the wall.
mouse
„Die Maus“ and „das Mäuschen“ can both be affectionate pet names for a child or a female partner. „Mäuschen“ is the more diminutive and especially intimate form, while „Maus“ sounds slightly less diminutive. Both words are possible; the choice mainly shows the desired degree of tenderness.

Komm, mein Mäuschen, wir gehen jetzt nach Hause.
Come on, sweetheart, we're going home now.
Komm, meine Maus, wir gehen jetzt nach Hause.
Come on, sweetie, we're going home now.