Either a ship designed to break through ice in frozen waters, or a conversation starter used to make people feel comfortable meeting each other.
Compound word combining 'ice' and 'breaker'; literally a boat that breaks ice (1880s), later metaphorically applied to social situations where someone 'breaks' the awkward silence.
The first icebreaker ships were so powerful they'd ram ice repeatedly until it shattered—and now we use the same word when someone tells a joke at a party, which is basically the social version of ramming through resistance.
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