To stick or glue things together into a mass; in linguistics, to build words by joining separate parts together.
From Latin agglutinare: ad- 'to' + glutinare 'to glue' from gluten 'glue.' Literally means 'to glue to something.'
Turkish and Finnish are agglutinative languages—a single word can be a whole sentence because you just keep sticking grammatical pieces together like building blocks, which is why Finnish words can look impossibly long to English speakers.
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