I have some fun etymological trivia related to that.
Late Proto-Indo-European had a root typically reconstructed as *kakka-. It means "to shit" or "shit". It's imitative in nature so not exactly a "fancy" word, right off the bat; more like a "child-friendly vulgarism".
That root is still present in Armenian (k'ak'), Russian (kakat'), Lithuanian (kaka), the Romance languages (cacare/cagar/etc.), the Iranian languages (kaka/kakā/kake/etc.), and other Indo-European languages. Still ranging from childish to vulgar.
A word that stops being used is not inherited. If that root was inherited by so many languages, it means that it kept being used, from ~five millenniums ago to now.
Obligatory musical reference: this goliardic song. "Oh, how beautiful it is, to shit [cagar] in the mountain, where the grass tickles you in the hole of the butt". It's just keeping up with a 5000yo tradition.
I think that it's a parallel development. It's unlikely to be a borrowing from some PIE descendant because
Proto-Germanic shifted PIE *k into *h (Grimm's Law), so the word would end as *hahha. Plus a direct descendant of the word isn't even attested in Germanic languages [see note].
Proto-Balto-Slavic and its descendants show a single consonant in that word, as PBS *kākā́ˀtei (see Latvian kakāt, Russian какать/kakat'). The result would be *kaka or *kakaa. (A double consonant often becomes single, but the opposite is rarely true.)
*NOTE: before someone mentions German "kacken", it's likely a borrowing from Latin "cacō" I shit. Now that's some borrowed shit!