/ˌmɒn.əˈlɪŋ.ɡwəl/
[adjective] Able to speak or use only one language. It can also describe a text, dictionary, or community that uses just one language.
The 'mono' in monolingual comes from a Greek word meaning 'alone' — making every monolingual speaker, by definition, linguistically lonely.
The word 'monolingual' is a fairly young word in English, appearing in the mid-to-late 19th century, but its two building blocks are ancient and come from very different corners of the classical world. The first half, 'mono-', is a prefix borrowed from Ancient Greek 'monos', meaning 'single', 'alone', or 'only'. The second half, '-lingual', comes from the Latin word 'lingua', which meant both 'tongue' (the physical organ in your mouth) and 'language' (the thing that tongue produces). The combination is a classic example of what scholars call a Greco-Latin hybrid — a word assembled from Greek and Latin parts, a very common practice in the scientific and academic coinage of the 18th and 19th centuries.
The Greek root 'monos' has remarkable staying power in English. It travelled through Ancient Greek texts and entered scholarly Latin, then moved through Old French and eventually into English across many centuries, giving us a huge family of 'mono-' words. The chain looks like this: Ancient Greek 'monos' → Medieval Latin 'mono-' (used as a prefix in learned writing) → Old French 'mono-' → Middle and Modern English 'mono-'. From this same root we get 'monastery' (a place for a person living alone), 'monotone' (a single, unchanging tone), and 'monopoly' (control by a single seller, from Greek 'monos' + 'polein', meaning 'to sell').
The '-lingual' half has an equally rich story. Latin 'lingua' — meaning tongue and language — is believed to descend from an even older Proto-Indo-European root, something like '*dnghwéh₂s', which also meant tongue. This ancient root is the common ancestor of a huge range of words across European languages. In Latin, 'lingua' produced 'lingualis' (relating to the tongue), and from there English borrowed many words directly or through French: 'language' came via Old French 'langage' from Latin 'lingua'; 'lingo' is a casual borrowing of 'lingua' itself; and 'linguist' (a person who studies or speaks languages) came from Latin 'linguista' in the 16th century.
When 'monolingual' was put together in the 19th century, the world was becoming more aware of multilingualism as a concept, particularly in the growing fields of linguistics, anthropology, and colonial administration, where European powers encountered and documented peoples speaking many different languages. The word was needed to describe the opposite situation — a person or community confined to a single tongue. It joined a neat family of parallel coinages: 'bilingual' (two languages, from Latin 'bi-', meaning two), 'trilingual' (three), and eventually 'multilingual' (many, from Latin 'multus', meaning much or many).
What makes 'monolingual' quietly fascinating is that for most of human history, truly monolingual people were probably the exception rather than the rule. Most communities lived on borders, trade routes, or in contact with neighbouring groups, and speaking at least some of a second tongue was simply practical survival. The concept of monolingualism as a norm — or even an ideal — is largely a product of modern nation-states, which promoted a single 'official' language as a symbol of national identity. In that sense, the word 'monolingual' was almost invented to describe a condition that nationalism was, paradoxically, in the process of creating.