Where does “thralle” come from?

thralle (Middle English) comes from Old English þrǣl, from Old Norse þræll, from Proto-Germanic þragilaz, from Proto-Indo-European trāgʰ- — pull, drag, race, run; to pull, drag, race, run.

Ancestry of “thralle”, step by step

Step	Language	Word	Meaning
1	Old English	þrǣl	thrall, slave, servant
2	Old Norse	þræll	slave
3	Proto-Germanic	þragilaz	runner; gofer; servant
4	Proto-Indo-European	trāgʰ-	pull, drag, race, run; to pull, drag, race, run

Every word from Proto-Indo-European trāgʰ- →Every word from Proto-Germanic þragilaz →Every word from Old Norse þræll →