verpennen
to oversleep
(Colloquial) To sleep in; to not wake up on time and therefore miss an appointment or obligation.
Mist, ich habe total verpennt und den Wecker nicht gehört!
Damn, I totally overslept and didn't hear the alarm!

to oversleep / miss
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to oversleep
(Colloquial) To sleep in; to not wake up on time and therefore miss an appointment or obligation.
Mist, ich habe total verpennt und den Wecker nicht gehört!
Damn, I totally overslept and didn't hear the alarm!

The verb is made up of the prefix 'ver-' and the colloquial word 'pennen' (to sleep). The prefix 'ver-' here signals a mistake or omission. Because of 'pennen', the word is very informal.
Ich bin gestern auf dem Sofa eingepennt.
I dozed off on the sofa yesterday.
'Verpennen' is an inseparable verb because it has the prefix 'ver-'. The prefix remains firmly attached to the verb in all tenses and sentence constructions.
Er hat den wichtigsten Teil des Films verpennt.
He missed the most important part of the film.
In the meaning of 'etwas versäumen' (to miss something), 'verpennen' is always used with an accusative object. Typical things that one 'verpennt' (misses) are appointments, deadlines, opportunities, or important events.
Leider habe ich die Chance auf die günstigen Tickets total verpennt.
Unfortunately, I totally missed the chance to get the cheap tickets.
Whether 'verschlafen' (to oversleep) or 'etwas versäumen' (to miss something) is meant can often be recognized from the object. If there is no object, or if it refers to sleeping ('den Morgen verpennen' — to sleep through the morning), meaning 1 is intended. With a specific event as the object ('den Termin verpennen' — to miss the appointment), meaning 2 is intended.
Ich habe heute Morgen total verpennt und bin zu spät gekommen.
I totally overslept this morning and arrived late.
to forget
'vergessen' and 'verpennen' can both mean that you do not do something on time or that you miss it. 'vergessen' is neutral: you did not think of doing or taking something. 'verpennen' is colloquial and sounds more casual. It often means that you overslept and therefore arrive late. But it can also mean that you missed something because you were careless. The words can be confused because both can lead to a missed appointment. If sleep is not involved, or if you want to speak neutrally, 'vergessen' fits better.

Am Montag verpennt du den Bus, wenn du den Wecker wieder so spät stellst.
On Monday you'll miss the bus if you set your alarm that late again.
Zu Hause habe ich gemerkt, dass ich meine Einkaufsliste vergessen habe.
At home I realized that I had forgotten my shopping list.