Slavery or captivity, or to keep someone enslaved or completely under your control.
From Old Norse 'þræll' meaning slave or servant. It entered English during Viking contact periods and originally meant a person of unfree status, later becoming a metaphor for any complete loss of freedom.
The word gives us insight into Viking society—'thralls' were a recognized social class, and Norse culture is often cleaner in historical memory than it should be, since slavery was absolutely central to Viking economics and expansion.
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