Solipsism is the philosophical concept that only your own mind definitely exists, and that any other knowledge is uncertain. The idea was first put forth by Descartes and his statement cogito, ergo sum, but the term wasn't borrowed into English until 1871. The word is composed of three parts, all in Latin: solus, meaning "alone", ipse, meaning "self", and -ism, a suffix commonly used for ideologies or concepts. Solus, which is the etymon of English sole ("alone"), comes from the Proto-Indo-European reflexive pronoun swe, which can sort of be translated as "oneself". Ipse, since it also has to do with the concept of self, coincidentally also comes from swe, and -ism has an uninteresting history going back to Ancient Greek. That redundant etymology of solipsism is quite interesting: oneself-oneself-ism!
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.