poppen
to fuck
A colloquial expression, often perceived as vulgar, for having sexual intercourse.
Die beiden haben sich auf der Party kennengelernt und später miteinander gepoppt.
The two met at the party and later fucked.

to fuck / pop
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to fuck
A colloquial expression, often perceived as vulgar, for having sexual intercourse.
Die beiden haben sich auf der Party kennengelernt und später miteinander gepoppt.
The two met at the party and later fucked.

The three meanings are clearly separated by context. The vulgar usage refers to people, the sound refers to objects (such as bottles), and the appearance refers to computer elements.
Auf dem Bildschirm poppte plötzlich eine Warnung auf.
A warning suddenly popped up on the screen.
In the meaning 'Geschlechtsverkehr haben' (to have sexual intercourse), 'poppen' (to fuck) is very colloquial and is often perceived as vulgar or at least very casual. It should absolutely be avoided in formal situations.
Im Film reden die Jugendlichen sehr direkt übers Poppen.
In the film, the teenagers talk very directly about fucking.
In the meaning 'knallen' (to pop/bang), 'poppen' (to pop) is an onomatopoeic word. It imitates the short, bright sound it describes, most commonly when opening a bottle of sparkling wine.
Wir hörten es in der Küche laut poppen und wussten, der Sekt ist offen.
We heard a loud pop in the kitchen and knew the sparkling wine was open.
In a computer context, the word is very often used as the separable verb 'aufpoppen' (to pop up). It then means that something suddenly appears on the screen.
Immer wenn ich diese Webseite besuche, poppt ein Werbefenster auf.
Whenever I visit this website, an advertising window pops up.
to appear
'erscheinen' and 'poppen' can both describe something becoming visible. 'erscheinen' is general and neutral: something comes into view or becomes visible. 'poppen' is used in this meaning mainly with computers, phones, or websites, when a window or message suddenly appears. It is colloquial and often occurs in the form 'aufpoppen' or 'poppt auf' (pops up). Outside of technology, 'erscheinen' is usually more appropriate.

Beim Öffnen der Webseite poppt sofort ein Werbefenster auf.
When you open the website, an advertising window immediately pops up.
Nach ein paar Sekunden erscheint das Bild auf dem Bildschirm.
After a few seconds, the image appears on the screen.