means A thin strand of spun fiber used for sewing or weaving, or by extension any continuous line of connected things — a sequence of messages, thoughts, or events.

from From Old English 'thrǣd,' meaning a twisted filament, built on the root of 'thrawan,' to twist or turn (the same ancestor that gives us 'throw'). The image is literal: thread is fiber that has been twisted into strength. That twisting metaphor stretched naturally — a 'thread' of conversation, a 'thread' of an argument — and in the digital age slid easily into the chain of replies that keep a discussion twisted together.