Csdrhrt

I am wondering if there's a verb that means "listening stealthily" or "listening to someone or a group of people without having him/them know".

I am highly confident that there's a verb, but I can't seem to remember what it was.

Just so that we're in the same page, here's an example:

John put his ears over the door to ___ at the private conversation between the king and the queen.

The word you're looking for is probably eavesdrop:

Secretly listen to a conversation.
‘my father eavesdropped on my phone calls’

_{(source: Oxford Dictionaries)}

@Zikato reminded me of this scene from The Fellowship of the Ring where Sam is caught eavesdropping (around 2:45) on a secret conversation between Gandalf and Frodo.

While I think eavesdrop is probably your best choice, spy would also be an appropriate descriptor for the action of stealthily listening though it does include all kinds of observation, so watching (and I suppose smelling?) as well.

Observe (someone) furtively.

(Source: Oxford Dictionary )

You can use to listen in.

It can be used either without an object, or with one. In that case, you listen in on something.

Definition and examples from the Oxford dictionary:

listen in

Listen to a private conversation, especially secretly.

• Seth listened in as his father chatted philosophy with friends


• I hurried him into my room and shut the door behind us; I didn't want any eavesdroppers listening in on our conversation.


• At this point, I decided to stop listening in on their private conversation.

This is more of a visual, (perhaps even nasal), term, but it doesn't rule out listening:

snoop

intransitive verb

: to look or pry especially in a sneaking or meddlesome manner

She locks up her diary to keep her brother from snooping.

-- MW

A word that means "listening to someone or a group of people without having him/them know" but not "listening stealthily" is overhear

Hear (someone or something) without meaning to or without the knowledge of the speaker.
‘I overheard two doctors discussing my case.’

(source: usage from collins and definition from oxford)

I agree that eavesdrop is a good word for this.

An alternative synonym is earwig

Oxford Dictionaries definition

British informal

Secretly listen to a conversation.

‘he looked behind him to see if anyone was earwigging’

In this case ‘spy’ could do as well, though that indicates more than just listening, it’s more ‘observing stealthily’