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 flippant
Pronunciation: ˈflipənt
adjective
not showing a serious or respectful attitude

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: