пятно́ (Russian) comes from Old East Slavic пятьно, from Proto-Slavic pętьno, from Proto-Slavic pęta, from Proto-Slavic pęti, from Proto-Balto-Slavic pínˀtei, from Proto-Indo-European (s)penh₁- — to twist; to weave.
пятно́ (Russian): spot, mark