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Latin: cacare (to shit)

German: Kacke (the ck as usual indicates that this word may only be used with the voice raised beyond 85 dB and with at least 5 % spit by volume mixed into the air-stream).

And Tamil which is not a Romance language.

It has Indo-European roots https://lrc.la.utexas.edu/lex/master/0821. Although Tamil is Dravidian, probably a loan word.

Since Tamil has borrowed more Dutch words (and uses at least one from Tamil, being katamaran), I expect your kak to come from the dutch kak, as kak always rolls down hill.

In greek as well.