There's no reason 'dog' means dog — Saussure said that's true of every word

Linguist Ferdinand de Saussure argued that the link between a word (the 'signifier') and what it refers to (the 'signified') is entirely arbitrary — nothing about the sound 'dog' resembles a dog, and other languages prove it by using completely different sounds for the same animal. The connection only holds because a community of speakers agrees to it by convention. The idea became a foundation stone of modern linguistics.

— Ferdinand de Saussure, Course in General Linguistics — Compiled from lecture notes by students, published 1916
Go deeper: get the book →