Cap
cap; a lie / 'no cap' = no lie, for real (anglicism from US rap slang)
Origin
Anglicism from African-American English (AAVE). As early as the 20th century 'to cap' meant 'to boast, exaggerate, fib' in Black US communities (part of the rap-battle tradition 'The Dozens'). Through 2010s hip-hop lyrics (e.g. Young Thug, Migos) and, from 2020, TikTok/Twitter it spread worldwide. 'Cap' = a lie, 'to cap' = to lie; the cap emoji 🧢 ironically marks a lie. Closely related to the common 'No Cap' (= no joke, for real).
Examples
Das ist Cap, das glaub ich dir nicht.
That's cap, I don't believe you.