Unfortunately (or perhaps thankfully) the words “random” and “randomly” are not used nearly as casually in German as they are in English nowadays. Germans say “zufällig” for “coincidentally” and “willkürlich” for “arbitrarily” to cover much of the ground of randomness:

– “Ich habe Franz zufällig gestern getroffen.” – “I randomly/coincidentally ran into Franz yesterday.”
– “Es war eine willkürliche Entscheidung.” – “It was a random/arbitrary decision.”

When deciding which word to use, it’s probably best to first consider what the English equivalent “random” is really trying to express. In most cases certain English-speakers (these days) use “random” to express that something is “unexpected” (“unerwartet”) or “weird” (“komisch”/”seltsam”). In German the terms “zufällig” and “willkürlich” still conform to their original intent.

Relatedly, the word for “random generator” is “Zufallsgenerator” (m.), and one would express “at random” with “aufs Geratewohl”.

Note: To really nail down English-speaking teenagers and college students’ use of the word “random” in German, one might use a construction with “irgend-“, best coupled with a word meaning “weird”:

– “Ich habe in irgendeiner komischen Kneipe irgendeinen komischen Typen getroffen.” – “I met a random (weird) guy in a random (weird) bar.”