There are 2 definitions of -in in English:

-in1

suffix

Chemistry
Forming names of organic compounds, pharmaceutical products, proteins, etc. insulin penicillin dioxin

Origin

alteration of -ine4.

Definition of -in in:

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Word of the day antebellum
Pronunciation: ˌantēˈbeləm
adjective
occurring or existing before a particular war…

There are 2 definitions of -in in English:

-in2

combining form

Denoting a gathering of people having a common purpose, typically as a form of protest: sit-in sleep-in love-in

Definition of -in in: