The thread of destiny- i.e. that on which destiny depends. The Greeks and Romans imagined that a grave maiden called Clotho spun from her distaff the destiny of man, and as she spun one of her sisters worked out the events which were in store, and Atropos cut the thread at the point when death was to occur.
A St. Thomas's thread.
The tale is that St. Thomas planted Christianity in China, and then returned to Malabar. Here he saw a huge beam of timber floating on the sea near the coast, and the king endeavouring, by the force of men and elephants, to haul it ashore, but it would not stir. St. Thomas desired leave to build a church with it, and, his request being granted, he dragged it easily ashore with a piece of packthread. (Faria y Sousa
Chief of the Triple Thread.
Chief Brahmin. Osorius tells us that the Brahmins wore a symbolical Tessera of three threads, reaching from the right shoulder to the left. Faria says that the religion of the Brahmins proceeded from fishermen, who left the charge of the temples to their successors on the condition of their wearing some threads of their nets in remembrance of their vocation; but Osorius maintains that the triple thread symbolises the Trinity.
“Terna fila ab humero dextero in latus sinistrum gerunt, ut designent trinam in natura divina rationem.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894