From Middle English oter, otir, otur, otyre, from Old English otor, from Proto-West Germanic *ot(t)r, from Proto-Germanic *utraz, from Proto-Indo-European *udrós, from Proto-Indo-European *wed-.
Cognate with Saterland Frisian Otter, Dutch otter, German Otter, Swedish utter, Norwegian oter, Icelandic otur, Sanskrit उद्र, Russian вы́дра, and Ancient Greek ὕδρα. More etymology under English water.