der Mumm
guts
A colloquial expression for courage, determination, or nerve to do something difficult, unpleasant, or risky.
Sie hatte nicht den Mumm, ihm die Wahrheit zu sagen.
She didn’t have the guts to tell him the truth.

guts / courage
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guts
A colloquial expression for courage, determination, or nerve to do something difficult, unpleasant, or risky.
Sie hatte nicht den Mumm, ihm die Wahrheit zu sagen.
She didn’t have the guts to tell him the truth.

'Mumm' (guts / courage) is a colloquial word for courage. It is used mainly in spoken language and in informal contexts.
Wow, du hast echt Mumm, das zu tun!
Wow, you really have guts to do that!
'Mumm' (guts / courage) is very often used in the construction 'den Mumm haben, etwas zu tun' (to have the guts to do something). It is followed by an infinitive clause with 'zu'.
Er hatte nicht den Mumm, seinen Chef nach einer Gehaltserhöhung zu fragen.
He didn’t have the guts to ask his boss for a raise.
A fixed expression is 'Mumm in den Knochen haben' (to have guts in one’s bones). It means that someone is brave, energetic, and not easily intimidated.
Die alte Dame hatte noch richtig Mumm in den Knochen und sagte allen ihre Meinung.
The old lady still had real guts and told everyone what she thought.
courage
'der Mut' and 'der Mumm' both mean that someone dares to do something difficult or unpleasant. 'Mut' is neutral and fits almost any context, including serious or formal situations. 'Mumm' is colloquial and sounds more direct, sometimes also a bit cheeky or casual. For example, you can say that someone has 'den Mumm' (the guts) to say something openly. For exams, conversations, or serious texts, 'Mut' is usually more natural. Among friends, 'Mumm' can work well.

Hast du den Mumm, dem Chef deine Idee direkt zu erklären?
Do you have the guts to explain your idea directly to the boss?
Das Kind zeigt Mut, als es allein zum Arzt hineingeht.
The child shows courage when they go into the doctor's office alone.